IMARCS staff course · Module 5

Salt Water, Salinity and Temperature

What salinity is, what changes it, how new salt water is made and matched, and why temperature matters.

1. Salinity
2. What changes salinity, and what doesn't
3. Making new salt water
4. Matching new water to the system
5. Temperature
6. Small numbers, big reactions

1. Salinity: how salty the water is

Salinity is the amount of salt in the water. It's written in ppt, parts per thousand: grams of salt in every 1,000 grams of water. Natural seawater is about 35 ppt. Some tools show specific gravity (SG) instead. 35 ppt is about 1.026 SG. IMARCS ranges are written in ppt.

IMARCS LIVABLE RANGE. Salinity 30 to 35 ppt. Temperature 26 to 30 C. Nothing inside these ranges is an emergency. RACK-1 runs at 30 ppt, so 29 to 31 ppt is normal there.

Salt water isn't just salt. It's a mix of many things in the right amounts (Module 13, section 1). That's why salt water is made from a proper aquarium salt mix and RO water, never from table salt.

IF The system reads 33 ppt

THEN Inside the livable range. Not an emergency.

IF A G2N system reads 29 ppt

THEN Below the livable range. Retest, then follow the System Logs form.

IF A tool shows 1.026

THEN That's specific gravity, about 35 ppt.

Think it throughTwo staff members report the same system: one says 1.025, the other says 33 ppt. Is one of them wrong?

Not necessarily. They may be using different units. 1.025 SG and 33 ppt are close to the same salinity. Always check the unit before you compare numbers.

Remember: Salinity 30 to 35 ppt. Temperature 26 to 30 C. Inside those ranges, stay calm.

2. What changes salinity, and what doesn't

Salinity only changes when the amount of water changes, or the amount of salt changes. Every time salinity moves, one of these two things happened. Find which one.

What happened	Salinity	Why
Evaporation	Goes up	Only water leaves. The salt stays.
ATO adds RO water	Goes back down	Water is added back. No salt is added.
Salt water taken out, ATO refills with RO	Goes down	Salt left with the water. Only water came back.
A slow leak, with the ATO on	Goes down slowly	Salt water leaks out. The ATO replaces it with RO water.
Skimmer cup collects a lot of water	Goes down slowly	That water is salt water. The ATO replaces it with RO water.
Salt water put in the ATO tank	Goes up every day	Salt is added with every top-off.
Water change with new water at the same salinity	Stays the same	Same amount of water and salt in, as out.

IF Salinity goes up

THEN Water left, or salt was added. Check the ATO, the ATO tank and what went into the ATO tank.

IF Salinity goes down

THEN Salt water left and RO came back, or extra RO went in. Check for leaks, water taken out, the skimmer cup.

IF Salinity drops a little every day and the ATO tank empties fast

THEN Salt water is leaving somewhere and being replaced with RO. Look for a leak or an overflowing skimmer.

Think it throughA system's salinity drops 0.2 ppt every day for a week. No water changes were done. The ATO tank is emptying faster than normal. What's the most likely cause, and what do you check?

Salt water is leaving the system and the ATO is replacing it with RO water. Check for a leak (floor, stand, plumbing) and check the skimmer cup for lots of watery skimmate.

Think it throughSalinity on RACK-1 is going up a little every day. RACK-1 has no ATO. What should you check first?

Whether it's being topped off with RO water to the tape line every day. If the top-off is missed, evaporation pushes salinity up.

Remember: Salinity only moves when water or salt moves. Find which one.

3. Making new salt water

New salt water is made in a separate container, never in a system. The IMARCS order is below: the water is heated to the right temperature before any salt goes in.

NEVER ADD POWDERED SALT STRAIGHT INTO A SYSTEM. Salt only goes into new water, in a separate container.

IMARCS ORDER. 1. RO water. 2. Heat it to the right temperature. 3. Add the salt into the water. 4. Mix until clear. 5. Measure salinity and temperature before it's used.

IF Tap water is used to mix salt water

THEN Everything in the tap water goes into the system.

IF Water is poured onto a pile of salt

THEN Wrong order. Salt goes into the heated water.

IF Salt is added to cold RO water

THEN Wrong order. Heat the water to the right temperature first, then add the salt.

IF New water goes in without being measured

THEN Nobody knows its salinity or temperature until the animals feel it.

IF Powdered salt is added straight into a system

THEN That breaks an IMARCS rule. Salt only goes into new water in a separate container.

Think it throughThe new salt water looks clear and the salt bag says it mixes fast. Do you still need to measure it?

Yes. Clear doesn't tell you the salinity or the temperature. Only measuring does.

Remember: RO water, heat, then salt. Measure before it goes in.

4. Matching new water to the system

Animals live in the system's water. New water that's very different in salinity or temperature changes their water suddenly. So new water is measured and matched before it goes in.

IMARCS RULE. On water change Fridays, measure the new salt water's salinity and temperature before it goes in.

A difference of up to 1 ppt between the new water and the target isn't a big deal. Don't chase small differences. Module 26 covers which differences need a slow acclimation when animals move.

IF New water is 0.5 ppt from the system

THEN Close enough. It can go in.

IF New water is much colder than the system

THEN The system temperature drops suddenly. Heat the new water first.

IF You skip measuring because the last batch was fine

THEN This batch is a new batch. Measure it.

Think it throughThe new water is 34.4 ppt and the system is 34.0 ppt. A coworker wants to add RO to the new water to match exactly. Good idea?

Not needed. 0.4 ppt is well within 1 ppt. Adding RO risks overshooting and makes more work. Close is good.

Remember: Measure the new water. Match it. Then it goes in.

5. Temperature

Temperature affects how much oxygen the water can hold. Cold water can hold more dissolved oxygen than warm water. Warmer water holds less.

Temperature also changes how dense water is. That's why the same water can read differently on tools that measure density (hydrometers) at different temperatures. Module 7 covers measuring.

IMARCS RULE. Temperature is always a manual reading, every working day, every tank, by hand with the meter in the water. Never copy the number off an Inkbird controller or the aquarium computer: if one of them or its probe fails, its number is wrong too.

Two devices should roughly agree

Your hand meter and the Inkbird or aquarium computer on that tank should read approximately the same. If there's a big difference, one of them is wrong. You can't tell which one by looking. Both have to be checked to find out which is incorrect.

IF The water gets warmer

THEN It holds less oxygen.

IF You copy the temperature from the Inkbird display

THEN If the probe has failed, you log a wrong number and nobody notices.

IF Your meter and the Inkbird disagree by a lot

THEN One of them is wrong. Check both to find which one.

IF The reading is 27.5 C

THEN Inside the livable range. Not an emergency.

Think it throughThe Inkbird display says 27.0 C. Your meter in the water says 30.8 C. Which number goes in the log, and what does the difference tell you?

The meter reading goes in the log, because temperature is always a manual reading. But a gap that big means one of the two devices is wrong, and you can't tell which by looking. Both have to be checked. If the water really is 30.8 C, it's above the livable range, so follow the System Logs form.

Remember: Read temperature by hand. It should roughly match the Inkbird. A big gap means check both.

6. Small numbers, big reactions

A small difference isn't a problem. Reacting too hard to it can create a bigger change than the one you were trying to fix.

Example: a system reads 0.3 ppt above target. That's well inside the livable range and well within 1 ppt. Adding salt or a lot of RO water to "fix" it creates a bigger, faster change than the one you were trying to fix.

IF A NUMBER IS INSIDE THE LIVABLE RANGE, IT ISN'T AN EMERGENCY. If a reading is far from the last one or doesn't make sense, test again before you write it down (Module 7). If it's still outside the range, follow the ranges shown on the System Logs form (the Log round form).

IF Salinity is 0.3 ppt above target

THEN Do nothing. Inside 1 ppt isn't a big deal.

IF Someone adds salt to fix a tiny difference

THEN A bigger, faster change than the one they tried to fix.

IF A reading makes no sense

THEN Test again before you write it down.

Think it throughSalinity is 34.6 ppt against a target of 34.0. What do you do?

Log it. It's inside the livable range and within 1 ppt. No action is needed.

Remember: Don't chase small numbers. Big fast changes do the damage.

7. What you see, and what it means

Salinity a little higher than yesterday	Evaporation. Check the ATO and the ATO tank (or the RACK-1 top-off).
Salinity going up every day	ATO not working, ATO tank empty, or salt water put in the ATO tank.
Salinity going down slowly, ATO tank emptying fast	Salt water is leaving (leak or skimmer) and RO is replacing it.
Salinity lower after water was taken out	The ATO replaced salt water with RO water. More than 250 ml out: add the same amount first (Module 13).
New water reads very different from the system	Don't add it yet. Adjust it in its container, then measure again.
Meter and Inkbird or aquarium computer disagree a lot	One of them is wrong. Check both to find which.
Reading far from the last one	Test again before you write it down.
Salinity within 1 ppt of target	Fine. No action.

8. Words to know

Salinity	How much salt is in the water.
ppt	Parts per thousand. Grams of salt in 1,000 grams of water. Natural seawater is about 35 ppt.
Specific gravity (SG)	Another way to show salinity, by comparing the water's density to pure water. 35 ppt is about 1.026.
Salt mix	Aquarium salt made to copy natural seawater. Never table salt.
RO water	Pure water with everything taken out. Used to mix salt water and to top off.
Livable range	The IMARCS range that isn't an emergency: salinity 30 to 35 ppt, temperature 26 to 30 C.
Dissolved oxygen	Oxygen mixed into the water. Animals breathe it. Warm water holds less.
Refractometer	A tool that measures salinity by how light bends through a drop of water (Module 7).
Hydrometer	A tool that measures salinity by the water's density. Temperature changes its reading.

Sources

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

Staff Course