Core Water Chemistry

ppt (parts per thousand) is used for salinity (Module 5). ppm (parts per million) is used for things present in much smaller amounts, like calcium, magnesium, nitrate and phosphate. In water, 1 ppm is about the same as 1 mg/L.

Alkalinity has two common units. dKH is used on many test kits. meq/L is used by others. 1 meq/L is about 2.8 dKH, so 2.5 meq/L and 7 dKH are about the same water. Before comparing two numbers, check they're in the same unit.

Watch the decimal point. A phosphate reading of 0.2 ppm and one of 2 ppm are ten times apart. A big jump from last time usually means a test error: the wrong sample size, or a misread result. Test again before you write it down.

Chloride and sodium together make up over 90% of what's dissolved in seawater. Magnesium is the third most common. Calcium and carbonate are smaller parts, but they're the ones calcifiers take out every day (Module 3). Reef aquarium targets are often given as about 380 to 450 ppm calcium and 1,250 to 1,350 ppm magnesium.

Trace elements are substances present in tiny amounts, like iodine, strontium and iron. Small doesn't mean unimportant: animals use them, and in a closed system they can run low. Measuring them takes special tests, often sent to a lab (Module 7).

The normal range is where a reading should be. Inside it, log it. The call limits are wider. A reading outside normal but inside the call limits is logged and reported the same day. A reading outside the call limits is retested, and if it's confirmed, reported right away.

Each system's log form shows its own ranges. For example, alkalinity can be anywhere from 6 to 10 dKH, but the preferred normal range is 7.5 to 8.5 dKH. So 7.5 to 8.5 is normal, and 6 to 10 are the call limits.

Test at about the same time of day each time. Some readings, like pH, change through the day (section 4), so a morning reading can't be fairly compared with an afternoon one.

Carbon dioxide (CO2) dissolved in water makes it more acidic, so more CO2 means lower pH. With the lights on, corals, clams and algae use CO2 for photosynthesis, so pH rises. With the lights off, everything keeps giving off CO2 and nothing uses it, so pH falls (Module 3). A healthy system reads lower in the morning than in the afternoon.

Indoor air often holds more CO2 than outdoor air, especially in closed rooms, and the water takes in CO2 from the air above it. A closed-up room can mean lower pH. Some systems use CO2 scrubber media to take CO2 out of the air going into a skimmer (Module 16). Used-up media can look the same as fresh media, so it's changed on a schedule, not by how it looks.

If a pH pen or meter reads a little off a reference, its offset is written on a label on that pen. Apply the offset yourself before recording. Forms don't apply offsets automatically (Module 7).

Carbon in seawater moves between three forms: carbon dioxide, bicarbonate and carbonate. At lower pH, bicarbonate is the main form. At higher pH, there's more carbonate. Alkalinity is mostly the bicarbonate and carbonate, and it's what soaks up acid so pH doesn't drop.

Calcifiers build with carbonate. As pH falls, carbonate falls fast: each drop of 0.3 in pH halves the carbonate, so a full unit drop leaves about a tenth. At lower pH, calcifiers have to work harder to build.

Calcifiers take alkalinity out every day, so it has to be put back (Module 19). Alkalinity and nutrients are linked too: as a general rule, lower nitrate and phosphate levels need lower alkalinity.

Corals appear to use calcium and alkalinity in about a 1 to 1 ratio as they build. When calcium and alkalinity are both pushed high, the water can hold more than is stable. Some of it then comes out of the water on its own as solid calcium carbonate. This is called precipitation. Both readings drop together, and a white crust can build up on heaters, pumps and other equipment.

Magnesium helps prevent this. It gets built into the surface of calcium carbonate and slows more from forming. When magnesium runs low, calcium and alkalinity come out of the water more easily. Higher pH also makes precipitation more likely.

That's why these three are watched together, and why dosing more to "fix" a low number can backfire if another one is out of balance.

A dosing pump can hum, click and look completely normal while delivering nothing: a dosing line clogged at its end by precipitate, a line that has burst or come loose somewhere along the way so the liquid never reaches the system, an empty reservoir, an air bubble or a failed pump head all look the same from the pump. The reservoir level is one check, not proof. If it should drop a little every day and today it reads the same as yesterday, nothing is leaving the reservoir: report it the same day. A level that does drop doesn't prove the dose reached the system either, since a burst or loose line can leak on the way. Check the whole path: the reservoir level, the line along its length, the tip where it enters the water, and the test results. Dosing pumps are covered in detail in Module 20.

If dosing stops and nobody notices, calcifiers keep using calcium and alkalinity, and the numbers fall over days. If a dosing pump runs too much, they climb. Either way, the reservoir and the test results together tell you what happened.

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 ammonia into the toxic form, so the same ammonia reading is more dangerous in warmer water or at a higher pH.

In a working system, bacteria turn ammonia into nitrite, then into nitrate (Module 8). Nitrate leaves the water in two ways: water changes take it out, and where there's no oxygen, some bacteria turn it into nitrogen gas (section 10).

Zero is harmful. Organisms in the system use nitrate and phosphate as food. When they read zero, those organisms are going hungry. So a normal reading is detectable, not zero.

Phosphate is the form of phosphorus found in the water. Every living thing needs phosphorus: it's part of DNA, of the molecule that carries energy in every cell, and of every cell membrane. Too much phosphate fuels nuisance algae, and it slows calcifying corals taking up calcium, so they grow more slowly. That matters most in systems with animals that build shells and skeletons.

Phosphate can be brought down several ways: water changes, chemical media, biological methods, or a combination. Module 17 compares them. Whatever the method, change it slowly: dropping phosphate fast is a change too.

A deep sand bed is a thick layer of fine sand. Near the top, water moves through it and bacteria use oxygen. Deeper down, water barely moves, so used oxygen isn't replaced, and below a certain depth there's almost none. Some bacteria live there anyway, using nitrate instead of oxygen and turning it into nitrogen gas, which leaves the water. This is called denitrification.

Living things use nitrogen and phosphorus in a fairly steady proportion: in ocean plankton, about 16 nitrogen for every 1 phosphorus. This is the Redfield ratio. A deep sand bed pulls nitrate down but does nothing to phosphate, which keeps arriving with food. Nitrate falls, phosphate builds, and the balance moves far from 16 to 1.

In most systems that isn't a big problem. It matters in systems with calcifiers, where high phosphate does damage. Water changes take phosphate out.

Dissolved oxygen comes in at the water surface, helped by flow, skimmers and air pumps, and is used by every animal and bacterium all the time (Module 3). Warm water holds less (Module 5).

ORP (oxidation reduction potential) is measured in millivolts (mV) by a probe. It gives a broad sign of how well the water can break down organic material. In the ocean it mostly reads between 300 and 450 mV. Readings of 200 to 450 mV are often recommended for aquariums, but those numbers come from experience, not science.

What matters most is the trend. ORP changes through the day, like pH. A sudden drop is a warning sign: a dead animal rotting unseen, for example, pulls ORP down. More oxygen and skimming push it up. ORP probes get dirty and drift, so they have to be cleaned and calibrated. Ozone raises ORP sharply and is dangerous to people and animals (Module 21).

Dissolved organics come from food, fish waste and anything decaying. As they build up, they can tint the water yellow, and very poor water may smell. They also feed bacteria and lower ORP.

Skimmers pull out some organics (Module 16). Activated carbon adsorbs the substances that yellow the water, and is replaced regularly as it fills (Module 17). Water changes dilute everything. Water that's turned yellow tells you organics are building faster than they're being removed.

Animals can adapt, and even thrive, at 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.

Watch for drift too: a number creeping slowly in one direction over weeks. Each reading looks fine on its own. The log shows the trend. A slow drift is easier to correct gently, and to trace to its cause, when it's caught early.

Phosphate climbs, and calcifiers take up calcium more slowly (section 9). Growing more slowly, they use less alkalinity each day. The dosing pump keeps adding the same amount, so alkalinity climbs. The alkalinity reading looks like a dosing problem, but the cause is the phosphate. Cutting the dose without fixing the phosphate leaves the real problem in place.

pH climbs, and more of the carbon in the water is carbonate (section 5). Carbonate is what calcification and precipitation take out of the water, so alkalinity is used up faster. If the dosing stays the same, alkalinity falls even though nothing about the dosing changed.

A dosing line clogs at its tip with precipitate (section 7). Calcium and alkalinity stop being replaced, and alkalinity falls. With less buffer, pH runs lower and swings more, and calcifiers grow more slowly. Here the first reading you notice may be pH, three steps from the cause.

The water warms. It holds less oxygen while the animals use more (Module 5), and more of any ammonia is in its toxic form (section 8). Two problems arrive from one cause.

To use a chain, work backward. When a reading moves, ask what would have to change upstream to cause it, then check those things. Fix the cause, not just the last number in the chain.

Use the links to reason. If pH is low but alkalinity is normal, look at CO2: the room air, or used-up scrubber media. If pH and alkalinity are both falling, check whether alkalinity is being replaced: is the dosing actually going in? If calcium and alkalinity drop together and crust appears on equipment, think precipitation, and check magnesium. If the water warms, expect less oxygen and more ammonia in its toxic form.

This is the thinking loop from Module 1 applied to chemistry: observe the numbers, check them, list what could link them, and rule causes out one at a time.