Making and Managing Salt Water

Salinity is written in ppt, parts per thousand: grams of dry salt in every 1,000 grams of water. Natural seawater is about 35 ppt. Some tools show specific gravity (SG) instead, which compares the water's density with pure water. Seawater at 35 ppt is about 1.026 SG. Before comparing two numbers, check they're in the same unit.

Seawater isn't just salt. Sodium and chloride make up over 90% of what's dissolved in it, but the rest, like magnesium, calcium and potassium, matters to the animals too. That's why salt water is made from an aquarium salt mix, made to copy natural seawater, and never from table salt.

This course uses a livable range of 30 to 35 ppt for salinity and 26 to 30 C for temperature. A reading inside those ranges is normal work: log it and carry on. Some systems are kept at a set point inside the range, and the log form shows what's normal for each one.

A reading outside the range, or one that's very different from the last reading, needs checking before anyone acts on it. Test again, ideally with a second meter (Module 1: check the facts). If it's confirmed, act on it.

When water evaporates, only pure water leaves and the salt stays behind, so salinity rises. Adding fresh water brings it back down. Taking salt water out and replacing it with fresh water lowers salinity, because salt left and only water came back. Adding salt raises it.

That's why top-off water is always fresh, purified water, never salt water (Module 4). Topping off with salt water adds salt every time, and salinity climbs to dangerous levels.

Start with the direction. Rising salinity means water left without being replaced, or salt came in. Check that the top-off is working and has water in it, and what went into its reservoir. Ask whether anyone added salt.

Falling salinity means salt water left and fresh water replaced it, or extra fresh water went in. Look for a leak around the stand, the floor and the plumbing. Ask whether water was taken out for a task. Check the skimmer: a wet skimmer can carry a lot of salt water out. A top-off reservoir emptying faster than usual is a clue that something is leaving.

This is the thinking loop from Module 1. List the possible causes, then rule them out with checks, not guesses. Fixing the number without finding the cause just means fixing it again tomorrow.

A salinity meter measures how well the water carries electricity: the more salt, the better it conducts. A refractometer measures how much light bends through a drop of water, and should be checked with calibration solution before use. A hydrometer measures density, and because water gets less dense as it warms, its reading changes with temperature.

Whatever the tool, the same habit applies: if a reading looks wrong, or is far from the last one, measure again, and confirm with a second meter before you record it or act on it. Calibration, accuracy and testing routines are covered in Module 7.

The order used in this course: start with purified water (Module 4), heat it to the system's temperature, then add the salt into the water, never the water onto the salt. Mix with a pump in the container until the water is clear and no salt is left on the bottom. Then measure salinity and temperature.

Salt makers give their own instructions, and they don't all agree. Red Sea's instructions: mix at no more than 20 C, heat the water only after the salt has dissolved, and mix no longer than needed. Read the instructions for the salt you use. If they differ from the procedure you were taught, ask before changing it.

A cloudy, white mix is usually precipitation: part of the salt has come out of solution as a fine white solid. Red Sea says it's caused by overheating, over-mixing or too much salt, and that it's harmless to fish and corals. It still means the mix wasn't made right, so find out which step went wrong.

Salt still on the bottom means it isn't fully dissolved. A reading taken now will be wrong, because the salt on the bottom isn't in the water yet. Keep mixing, then measure.

Mixed salt water doesn't keep forever, and how long it keeps depends on the salt. Red Sea's standard salt keeps about 7 days in a sealed container in the dark, and its high-calcium salt shouldn't be stored more than 12 hours. Read the label for the salt you use.

Before new salt water goes into a system, measure its salinity and temperature, and the system's. A difference of up to 1 ppt isn't a problem. Don't chase an exact match: adjusting a batch over and over risks overshooting.

Temperature matters as much as salinity. Water much colder or warmer than the system changes the animals' water quickly. That's why the water is heated before it's used. Moving animals between waters that don't match is covered in Module 25.

Animals breathe oxygen dissolved in the water. Cold water can hold more dissolved oxygen than warm water. As water warms, it holds less, while the animals in it use more. So warm water is a double squeeze on oxygen, and a pump or air supply failing matters more in warm water.

Temperature also changes how fast things happen: warmer water speeds up the animals' metabolism, and the bacteria and algae in the system too. A steady temperature inside the range matters more than any one number.

Through the day, lights warm the water while they're on, and pumps and other equipment add heat all the time. The room matters most: water slowly follows the air around it.

In summer, a warm room pushes water temperature up. More water evaporates, so the top-off uses more water, and the water holds less oxygen. Air conditioning, fans across the water surface, shorter light periods and chillers are all used to keep water cool (Module 14).

In winter, heaters do more of the work. A heater that fails off in a cold room lets the water cool. A heater that sticks on can overheat it. Both are covered in Module 14.

Temperature is read by hand, with the meter in the water. Don't copy the number off a controller display: if the controller or its probe has failed, its number is wrong too, and copying it hides the failure.

Your hand reading and the controller should be about the same. A big difference means one of them is wrong, and you can't tell which by looking. Check both before acting. A controller probe that has slipped out of the water, or sits right next to a heater, reads the wrong temperature, and the controller then heats or cools the water based on a wrong number.

Say a system reads 0.3 ppt above its target. That's well inside the livable range and within 1 ppt. Adding fresh water or salt to "fix" it makes a bigger, faster change than the one you were fixing, and often overshoots the other way.

Animals cope well with a steady number that isn't perfect. What hurts them is fast change (Module 6). So inside the range, log it and watch the trend. If it keeps moving the same way, find the cause (section 4).