IMARCS staff course · Module 3

Saltwater Chemistry

What pH, CO2, alkalinity, calcium, magnesium, ammonia, nitrite, nitrate and phosphate are, how they connect, why stable beats perfect, and how to read the numbers on the form.

1. Reading the numbers
2. pH and carbon dioxide
3. Alkalinity, calcium and magnesium
4. Ammonia, nitrite and nitrate
5. Nitrate and phosphate
6. Deep sand beds and the Redfield ratio
7. Stable beats perfect
8. How the numbers connect

1. Reading the numbers: normal range and call limits

Every number on the System Logs form shows its normal range. Some also have call limits, which are wider. Where a reading lands tells you what to do next.

IMARCS RULE. Outside normal: the round is flagged, tell Michael today. Outside the call limits: measure again, then call Michael now. Never try to fix a reading yourself.

Test	Which systems, when	Normal	Call limits
Temperature	Every system, every working day	26 to 28 C (RACK-1: 28 to 30 C)	26 to 30 C
Salinity	Every system, every working day	32 to 35 ppt (RACK-1: 29 to 31 ppt)	30 to 35 ppt (RACK-1: 29 to 35 ppt)
pH	Every system, Tuesday and Friday	7.8 to 8.6	7.8 to 8.6
Alkalinity	RS200-2 Tuesday and Friday, CADE-800 every working day	7.5 to 8.5 dKH	6.5 to 10 dKH
Nitrate	RS200-2 and CADE-800, Tuesday	Detectable up to 30 ppm	None
Phosphate	RS200-2 and CADE-800, Tuesday	Detectable up to 0.2 ppm	None

The call limits for temperature and salinity are the livable range from Module 2. RACK-1 runs at 30 ppt, so its salinity numbers are different.

IMARCS RULE. All water testing happens between 13:00 and 18:00. If that's impossible, test as close to it as you can and write the actual time in the Observations box.

IF A display system reads 27.4 C

THEN Normal. Log it.

IF A display system reads 29.1 C

THEN Outside normal, inside the call limits. The round is flagged. Tell Michael today.

IF A display system reads 31.0 C

THEN Outside the call limits. Measure again, then call Michael now.

IF dKH reads 9.2

THEN Outside normal (7.5 to 8.5), inside the call limits (6.5 to 10). Tell Michael today.

IF A test is done at 09:00 instead of 13:00 to 18:00

THEN It can't be compared to the last reading. Write the actual time in Observations.

Think it throughSalinity on RACK-1 reads 29.4 ppt. Salinity on G2N-2 reads 29.4 ppt. Is the answer the same for both?

No. RACK-1 runs at 30 ppt, so 29.4 is normal there. On G2N-2, 29.4 is below the 30 ppt call limit: measure again, then call Michael now. The same number can mean different things on different systems.

Remember: Normal: log it. Outside normal: tell Michael today. Outside the call limits: measure again, call now.

2. pH and carbon dioxide

pH tells you how acidic or basic the water is. Seawater is basic, above 7. In an aquarium, the main thing that moves pH is carbon dioxide (CO2). More CO2 in the water makes it more acidic, so pH goes down. Less CO2 lets pH go up.

With the lights on, corals, clams and algae use CO2, so pH rises. With the lights off, everything keeps breathing out CO2 and nothing uses it, so pH falls. A healthy tank with nothing wrong reads differently in the morning and in the evening.

That's why tests are done in the same 13:00 to 18:00 window. A reading taken at a different time can't be compared to the last one: you can't tell a real problem from the time of day.

CO2 in the room

Indoor air often has more CO2 than outdoor air, especially in closed rooms. The water takes CO2 from the air above it, so a closed room can mean a lower pH.

CO2 media on the CADE-800

The CADE-800 skimmer pulls air through a reactor packed with CO2 absorbing media before that air goes into the tank. The media strips CO2 out of the air. When the media is used up, it stops absorbing, and the skimmer pushes CO2 straight back into the water. Used-up media looks exactly the same as fresh media, so it's changed every Friday, all of it, even if it looks fine.

PEN OFFSETS. If a meter or pen reads a little off, its offset is written on a label on that pen. Add the offset to what the pen shows and enter the corrected number. The form doesn't add anything (Module 4).

IF The lights go off

THEN CO2 builds up and pH drifts down overnight. That's normal.

IF The room is closed up and the air is stale

THEN More CO2 in the air, more in the water, lower pH.

IF The CO2 media isn't changed on Friday

THEN It may be used up and you can't tell by looking. CO2 goes back into the water.

IF A pen has an offset label and you enter what the pen shows

THEN The logged pH is wrong by the offset. Add the offset first, then enter the number.

Think it throughA coworker tests pH at 08:30 and gets 7.95. On Tuesday afternoon it was 8.15. They say pH is crashing. What do you say?

The readings were taken at different times of day, so they can't be compared. pH is lower in the morning on a healthy tank. Test again in the 13:00 to 18:00 window. 7.95 is inside 7.8 to 8.6 anyway.

Think it throughThe CADE-800 pH has been drifting lower for two weeks. Alkalinity is normal. The CO2 media looks the same as always. What could it be, and what's the trap?

More CO2 is getting into the water. The CO2 media is one possibility, and the trap is that used-up media looks exactly like fresh media. Check that it has really been changed every Friday. Room air is another possibility. Log the readings and tell Michael if pH goes outside normal.

Remember: More CO2, lower pH. Test pH in the same time window every time.

3. Alkalinity, calcium and magnesium

Alkalinity is the water's buffer: it holds pH steady when acid is added. The higher the alkalinity, the more it resists pH dropping. Alkalinity is measured in dKH.

Corals, clams and coralline algae build their skeletons and shells from calcium and carbonate. As they grow, they take calcium and alkalinity out of the water every day. Magnesium is used too. If nothing puts them back, the numbers fall.

Calcium and magnesium are two of the main parts of seawater. At 35 ppt, natural seawater has about 410 ppm calcium and about 1,300 ppm magnesium (Module 6, section 1). They aren't on the daily form.

AFR is a powder mixed with RO water and dosed by a pump. It supplies calcium, magnesium, alkalinity and trace elements. On the CADE-800 it's dosed from a 5 liter reservoir. RS200-2 also has AFR, but it doses only a few milliliters a day. Dosing pumps are covered in detail in Module 8.

Alkalinity and nutrient levels

Alkalinity and nutrients are linked. As a general rule, lower nutrient levels (nitrate and phosphate) need lower alkalinity.

IMARCS RULE. The CADE-800 AFR level must go down every day, about 0.1 L. The same number as yesterday means the doser isn't delivering: call Michael the same day. A doser can hum, click and look completely normal while delivering nothing at all.

THE dKH RETEST RULE. If dKH has moved more than 0.5 since the last test on the same tank, test it again before writing it down. Second test agrees: record it and tell Michael. Second test is close to the last one: the first test was bad, record the second.

IF The CADE-800 AFR level is the same as yesterday

THEN The doser isn't delivering. Call Michael the same day.

IF AFR stops for a week and nobody notices

THEN Corals and clams keep using calcium and alkalinity. dKH drops.

IF dKH moved 0.8 since the last test

THEN Test again before writing it down.

IF The AFR mix is made "a bit stronger to be safe"

THEN Wrong. Changing the mix changes the water chemistry for every animal in that tank.

Think it throughThe CADE-800 AFR reservoir read 3.4 L yesterday and 3.4 L today. The dosing pump is humming. Is it fine?

No. The level has to go down every day. The same number means the doser isn't delivering, even if it hums. Call Michael the same day. If nothing is done, the corals and the clam get nothing and nobody finds out until dKH has already dropped.

Remember: Corals and clams use calcium and alkalinity every day. AFR puts them back. Check that it's actually going in.

4. Ammonia, nitrite and nitrate

Fish waste, uneaten food and anything that dies and rots make ammonia. In a working system, bacteria turn ammonia into nitrite, then nitrite into nitrate. Module 5 covers the bacteria.

Ammonia comes in two forms. The un-ionized form (NH3) is about 100 times more toxic to fish than the ionized form (NH4+). Higher pH and higher temperature push more of the ammonia into the toxic form. The same ammonia reading is more dangerous in warmer water, or at a higher pH.

Where nitrate goes

Nitrate leaves the water in two ways. Water changes take it out. And where there's no oxygen, anaerobic bacteria turn nitrate into nitrogen gas, which leaves the water. Many IMARCS systems have deep sand beds, and they pull nitrate down.

How a deep sand bed removes nitrate

A deep sand bed is a thick layer of fine sand. Near the top, water moves through the sand, and bacteria there use oxygen. Deeper down, the water barely moves, so the oxygen that gets used up isn't replaced. Below a certain depth there's almost no oxygen at all.

Some bacteria can live there anyway. With no oxygen to breathe, they use nitrate instead. As they use it, the nitrate is turned step by step into nitrogen gas, which bubbles out of the sand and leaves the water. This is called denitrification. It only happens where there's no oxygen, which is why the sand has to be deep: a thin layer of sand has oxygen all the way through.

Ammonia and nitrite aren't on the daily form. Nitrate is (section 5).

IF A fish dies and isn't found

THEN It rots and makes ammonia.

IF Temperature and pH both go up

THEN More of the ammonia is in the toxic form.

IF Bacteria are working

THEN Ammonia becomes nitrite, then nitrate.

IF A system has a deep sand bed

THEN Anaerobic bacteria deep in the sand turn nitrate into nitrogen gas. Nitrate is pulled down.

Think it throughTwo tanks have the same small amount of ammonia. One is 26 C at pH 7.9. The other is 29 C at pH 8.4. Which is more dangerous for the fish?

The 29 C, pH 8.4 tank. Higher temperature and higher pH both push more of the ammonia into the toxic NH3 form.

Remember: Ammonia, then nitrite, then nitrate. Nitrate leaves with water changes, or as gas from a deep sand bed.

5. Nitrate and phosphate: food, in the right amount

Nitrate and phosphate build up from food and waste. On the reef systems, RS200-2 and CADE-800, they're tested every Tuesday.

Zero is harmful

Nitrate and phosphate aren't just waste. Organisms in the tank consume them as a food source. When they read zero, those organisms are starving. That's why 0 isn't normal.

What phosphate is

Phosphate is the form of phosphorus found in the water. Every living thing needs it. It's part of DNA, part of ATP, the molecule that carries energy in every cell, and part of every cell membrane.

Why too much phosphate is bad

Too much phosphate fuels algae, which can take over a tank. It also slows calcifying corals taking up calcium, so they grow more slowly. That matters most in aquariums with animals that build shells and skeletons.

IMARCS RULE. Phosphate is removed with water changes. As a general rule, IMARCS doesn't use phosphate binding material.

IMARCS RULE. Nitrate: normal is detectable up to 30 ppm. Phosphate: normal is detectable up to 0.2 ppm. A reading of 0 (not detectable) isn't normal: tell Michael today.

Watch the decimal point. 0.2 ppm phosphate is normal. 2 ppm is ten times higher. A big change from last week usually means a test error: wrong sample size, or reading the color card wrong. Test again before you write it down.

IF Nitrate or phosphate reads 0

THEN Not normal: organisms that feed on it are starving. Tell Michael today.

IF Phosphate keeps climbing

THEN It fuels algae and slows calcifying corals. Phosphate comes out with water changes. Outside normal: tell Michael today.

IF Phosphate reads 2 when it was 0.08 last week

THEN Check the decimal point, then test again.

IF Nitrate reads 18 ppm

THEN Normal. Log it.

Think it throughA coworker says zero nitrate and zero phosphate is perfect, because they're waste. What do you say?

Zero isn't normal. Organisms in the tank consume nitrate and phosphate as a food source, so at zero they're starving. Tell Michael today.

Think it throughCADE-800 phosphate was 0.06 ppm last Tuesday. Today the card reads 0.6 ppm. What do you do before you write it down?

Check the decimal and test again. A tenfold jump in one week usually means a test error. If the retest agrees, record it: 0.6 is above 0.2, so it's outside normal. Tell Michael today.

Remember: Detectable, not zero. Nitrate up to 30 ppm, phosphate up to 0.2 ppm. Water changes, not binders.

6. Deep sand beds and the Redfield ratio

Living things use nitrogen and phosphorus in a fairly steady proportion. In ocean plankton it's about 106 carbon to 16 nitrogen to 1 phosphorus. This is called the Redfield ratio. The part to remember: about 16 nitrogen for every 1 phosphorus.

A deep sand bed pulls nitrate down, but it does nothing to phosphate. Phosphate keeps arriving with food and waste, so it builds up quickly. Nitrate goes down, phosphate goes up, and the balance between them is soon far from 16 to 1.

In most tanks this isn't a big deal. It matters in aquariums with calcareous organisms: corals, clams and other animals that build shells and skeletons. That's where high phosphate does damage (section 5), and water changes are how it comes back out.

IF A deep sand bed pulls nitrate down

THEN Phosphate isn't pulled down. It keeps building.

IF Nitrate falls and phosphate rises

THEN The balance moves far from 16 to 1.

IF The tank has corals and clams

THEN This is where high phosphate matters most.

Think it throughA tank with a deep sand bed shows nitrate falling week after week while phosphate climbs. A coworker suggests adding phosphate binder. What do you say?

The deep sand bed is pulling nitrate down and doing nothing to phosphate, so the balance is moving away from 16 to 1. IMARCS doesn't use phosphate binding material as a general rule: phosphate comes out with water changes. Log the numbers and tell Michael if they're outside normal.

Remember: Deep sand beds take nitrate out, not phosphate. Water changes take phosphate out.

7. Stable beats perfect

A number that isn't ideal but is stable is far less harmful than constantly chasing the ideal number and causing swings. Animals can adapt, and even thrive, under numbers that aren't ideal, as long as those numbers are stable.

Every "fix" is a change. Fixing a small difference today, then fixing the overshoot tomorrow, gives the animals a new number every day. That's what does the damage. This is also why you never try to fix a reading yourself: log it, and follow the form.

IF A number is a little off ideal but steady for weeks

THEN Stable. Don't chase it.

IF Someone "fixes" a small difference every day

THEN The animals get a new number every day. The swings do the damage.

IF You want to correct a reading yourself

THEN Don't. Log it and follow the form.

Think it throughdKH on a tank has read 7.3 for three weeks. The animals look healthy. Someone wants to bring it up to 8.0 today. Good idea?

No. 7.3 is outside normal, so it's logged and Michael is told, but the number is stable and the animals are doing well. A sudden change is a bigger risk than a stable number that's a little low. Never fix a reading yourself.

Remember: Stable and not ideal beats ideal and swinging.

8. How the numbers connect

These numbers are linked. When one moves, others can move with it. Finding the cause means looking at all of them together, not one at a time.

IF pH is low but alkalinity is normal

THEN Look at CO2: the room air, and the CO2 media on the CADE-800.

IF pH and alkalinity are both falling

THEN Check whether alkalinity is being replaced: is the AFR actually going in?

IF The water gets warmer

THEN Less oxygen, and more of any ammonia is in its toxic form.

IF Nitrate low, phosphate high

THEN A deep sand bed is pulling nitrate. Phosphate needs water changes.

Think it throughRS200-2: pH 7.85 and dKH 6.9 on Friday. On Tuesday it was pH 8.10 and dKH 7.9. What's the first thing to check, and why?

Both are falling together, and alkalinity is what holds pH up. First retest dKH, since it moved more than 0.5. If it agrees, check whether alkalinity is being replaced: the RS200-2 AFR tube clear at the sump end, and the dosing level dropping. dKH 6.9 is outside normal: tell Michael today.

Remember: One number moving can move others. Look at what moved with it.

9. What you see, and what it means

pH lower in the morning than in the afternoon	Normal day and night cycle. Compare readings from the same time window.
pH low, alkalinity normal	More CO2: closed room air, or CO2 media used up (CADE-800).
pH and alkalinity both falling	Alkalinity isn't being replaced. Check the AFR is actually going in.
CADE-800 AFR level same as yesterday	The doser isn't delivering. Call Michael the same day.
dKH moved more than 0.5 since the last test	Test again before writing it down.
dKH 8.6 to 10, or 6.5 to 7.4	Outside normal, inside the call limits. Tell Michael today.
Nitrate or phosphate reads 0	Not normal: organisms that feed on it are starving. Tell Michael today.
Nitrate low, phosphate climbing week after week	A deep sand bed is pulling nitrate down while phosphate builds. Water changes take phosphate out.
Phosphate ten times higher than last week	Check the decimal, test again. If it agrees, it's outside normal: tell Michael today.
A number a little off ideal but steady for weeks	Stable. Don't chase it. Log it and follow the form.
A reading outside the call limits	Measure again, then call Michael now.

10. Words to know

pH	How acidic or basic the water is. Below 7 is acidic, above 7 is basic. Seawater is basic.
Carbon dioxide (CO2)	A gas that animals breathe out. In water it makes acid and lowers pH.
CO2 media	Material that absorbs CO2 from the air going into the CADE-800 skimmer. Used-up media looks the same as fresh.
Offset label	A label on a pen or meter that reads a little off. Add the offset to what it shows before you enter the number.
Alkalinity (dKH)	The water's buffer that holds pH steady. Used up by corals and clams.
Buffer	Something that resists a change. Alkalinity resists pH dropping.
Calcium	One of the main parts of seawater. Corals and clams use it to build skeletons and shells.
Magnesium	One of the main parts of seawater, about 1,300 ppm at 35 ppt.
AFR	A powder mixed with RO water and dosed by pump. Supplies calcium, magnesium, alkalinity and trace elements.
Ammonia	Waste from animals and rotting matter. Its NH3 form is very toxic.
Nitrite	The middle step: bacteria make it from ammonia, then turn it into nitrate.
Nitrate	The end step. A food source for organisms in the tank. Normal at IMARCS: detectable up to 30 ppm.
Phosphate	A nutrient every living thing needs. Too much fuels algae and slows calcifying corals. Normal at IMARCS: detectable up to 0.2 ppm.
Anaerobic	Without oxygen. Anaerobic bacteria turn nitrate into nitrogen gas.
Denitrification	Bacteria with no oxygen breathing nitrate instead, turning it into nitrogen gas that leaves the water.
Deep sand bed	A thick sand layer whose deep parts have no oxygen. It pulls nitrate down, but not phosphate.
Redfield ratio	The steady proportion of nitrogen to phosphorus in living things: about 16 to 1.
Normal range	Where a reading should be. Outside it: tell Michael today.
Call limits	The wider limits. Outside them: measure again, then call Michael now.

Sources

The quiz for this module opens when quiz grading goes live.

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