Reef Tank Chemistry 101

If you take one thing from that table, take this: nitrate and phosphate should not be zero. The old reefing wisdom of driving nutrients to undetectable was wrong, and tanks run that way go pale, get dinoflagellates, and lose colour. Corals need nitrogen and phosphorus. Their symbiotic zooxanthellae need them more.

Coral skeleton is calcium carbonate. To build it, a coral takes calcium ions and carbonate ions out of the water and lays them down.

That means every millimetre of new SPS growth removes calcium and alkalinity from your tank simultaneously, in a fixed ratio. Roughly, for every 1 dKH of alkalinity consumed, about 7 ppm of calcium goes with it.

That ratio is genuinely useful as a diagnostic. If your alkalinity drops 1 dKH per day and calcium drops around 7 ppm per day, that is normal coral growth and you dose to match. If alkalinity drops sharply and calcium does not move, something other than coral growth is consuming it, and precipitation onto heaters and pumps is the usual answer.

Magnesium is the third leg because it interferes with calcium carbonate crystal formation in the water column. Without adequate magnesium, calcium and carbonate precipitate out of solution before your corals can use them. You will find that no matter how much you dose, calcium and alkalinity will not both hold. Check magnesium first any time the other two refuse to stay up.

Magnesium is consumed slowly compared to the other two, and in a lightly stocked tank, water changes alone may keep up.

You cannot dose what you cannot measure, and reef test kits vary enormously.

Alkalinity is the parameter you will test most, so make it easy on yourself. A Hanna checker gives a number instead of a colour judgement and removes the argument with yourself about whether that is blue or blue-green. Salifert titration kits are accurate in good hands and cheaper.

Calcium and magnesium are titrations in most hobby kits. Slower, so most people test them weekly rather than daily.

Salinity needs a refractometer calibrated with 35 ppt calibration fluid, not with distilled water alone, and definitely not a plastic swing-arm hydrometer. Recalibrate monthly. A refractometer that has drifted will make you chase parameters that were never wrong.

ICP testing sends a sample to a lab and returns a full elemental profile including trace elements and contaminants you cannot test for at home. Worth doing two or three times a year, or immediately when a tank is behaving badly for no visible reason. Results vary between labs, so compare a tank to its own history rather than to someone else's report.

Establishing your consumption rate

This is the step people skip, and it is the whole basis of dosing.

Test alkalinity at the same time on two consecutive days without dosing anything. The difference is your daily consumption. Do the same for calcium over a week.

A brand new tank with a few frags consumes almost nothing. Do not start dosing on day one because a guide said to. You will overshoot and drive alkalinity up while nothing is using it.

A mature SPS-dominant tank might consume 1 to 2 dKH per day. That is a completely different dosing regime from a soft coral tank that consumes 0.2 dKH per day. Both are correct for their own tanks.

Water changes only

For soft corals, LPS at low density, and anything not growing fast, regular water changes with quality salt mix genuinely can be enough. A 10% weekly change replaces a meaningful share of what was consumed.

The moment your alkalinity drops measurably between water changes, this stops being sufficient.

Two-part dosing

The standard approach. Two solutions dosed separately:

• Part A: calcium chloride, raises calcium.
• Part B: sodium carbonate or bicarbonate, raises alkalinity.

Dose them at different times of day, or at minimum with several hours between, and into areas of high flow. Dosed together in the same spot, they precipitate into a cloud of calcium carbonate and you have wasted both.

Note the difference between the two carbonate options. Sodium bicarbonate (baking soda) raises alkalinity while nudging pH slightly downward. Sodium carbonate (soda ash, or baking soda baked in an oven) raises alkalinity and pushes pH upward. If your tank runs low pH, soda ash helps. If it runs high, bicarbonate is the safer choice.

A dosing pump splits the daily amount into many small doses across the day, which is far better than one large manual dose. Small, frequent, boring: that is what you want.

Two-part dosing adds sodium and chloride to the tank over time. In heavily dosed systems this drifts over months, which is one of the things ICP testing catches and one of the reasons regular water changes still matter.

Kalkwasser

Calcium hydroxide dissolved in RO water, usually dripped in through an auto top-off. It supplies calcium and alkalinity in the correct ratio in one solution, and it raises pH, which is useful in tanks that struggle with low pH from indoor CO₂.

The limitation is that saturated kalkwasser has a ceiling on how much it can deliver per litre of top-off. A high-demand SPS tank will outrun it. Many people run kalkwasser as the base and two-part on top.

It is also caustic. Mix carefully, keep it sealed, and never dump it in quickly.

Calcium reactor

Carbon dioxide dissolves aragonite media in a sealed chamber, and the resulting effluent carries calcium, alkalinity, and trace elements into the tank in natural ratio. Excellent for large or high-demand systems and effectively self-balancing once tuned.

It is more equipment, more failure modes, and requires CO₂ management. Not a starting point.

Here is where most reef losses happen. Not from the wrong number, but from correcting one too quickly.

Never raise alkalinity by more than about 1.4 dKH in a single day. Less is better. Half that is safer. A rapid alkalinity increase causes tissue burn and rapid tissue necrosis in SPS, and you can lose colonies over a well-intentioned correction.

Lowering alkalinity is done by simply stopping dosing and letting consumption bring it down, or by water changes with lower-alkalinity water. There is no safe fast method.

Calcium tolerates change better than alkalinity, but there is no reason to rush it either.

Magnesium can be raised relatively aggressively, up to around 100 ppm per day, without drama.

Salinity should move no faster than about 0.001 sg per day.

The general rule I would give anyone: if a parameter is wrong but the corals look fine, you have weeks to fix it. Behave accordingly. The single most damaging thing in reefing is an anxious person with a bottle of supplement making three corrections in a day.

People chase pH more than they should. In a closed home aquarium, pH is largely driven by dissolved CO₂, which is largely driven by the CO₂ in your house. A sealed home in winter with the heating on can sit at 800 to 1200 ppm atmospheric CO₂, and your tank pH will sit low as a result.

If pH is low but alkalinity is correct, the answer is more gas exchange, not chemicals. Options: run the skimmer air intake to a window or outside, add a CO₂ scrubber on the skimmer intake, increase surface agitation, or run a refugium on a reverse light cycle so photosynthesis is happening at night when the display is dark.

Do not dose pH-up products. They raise alkalinity as a side effect and you lose control of the parameter that actually matters.

The modern consensus is that reef tanks need measurable nitrate and phosphate.

Symptoms of nutrients too low: pale, washed-out coral colour, tissue recession at the base, dinoflagellate outbreaks, cyanobacteria. Fish looking thin because you have been underfeeding to keep numbers down.

Symptoms of nutrients too high: nuisance algae, brown coral colouration, cloudy water, reduced coral growth at the extreme end.

The gap between those two is wide, and the low end causes more tank crashes than the high end. If your nitrate reads zero, feed more. That is not a joke, it is the actual fix. If it stays at zero, dose nitrate directly.

Nitrate and phosphate are also linked. Driving one to zero while the other remains available creates the imbalance that dinoflagellates exploit, and dinos are considerably harder to get rid of than a bit of algae. Aim to keep both present, in a rough ratio around 100:1 nitrate to phosphate.

Daily: glance at the tank. Check temperature, check the return pump is running, check the auto top-off reservoir. Look at the corals rather than the numbers. Polyp extension tells you more than a test kit.

Every one to three days: alkalinity test if you are dosing. This is your early warning system for everything else.

Weekly: calcium, magnesium, nitrate, phosphate, salinity. Clean the skimmer cup. Water change 10 to 15%.

Monthly: recalibrate the refractometer. Clean pumps and powerheads. Replace filter socks or clean the roller.

Quarterly: ICP test if you use them. Deep clean of return pumps. Inspect and clean any dosing lines, which clog with precipitate over time.

• Dosing before measuring consumption. You will drive parameters up in a tank that was not using anything.
• Chasing a number you read on a forum. A tank at 7.8 dKH held perfectly does not need to be at 8.5.
• Testing at different times of day and comparing the results. Alkalinity consumption is not uniform across the day.
• Correcting two parameters at once. When something goes wrong afterward you cannot tell which change caused it.
• Ignoring magnesium for months. It drifts down slowly and quietly, and you will spend weeks confused about why calcium will not hold.
• Trusting a test kit that has expired. Reagents degrade. Write the open date on the box.
• Adding new corals during a parameter correction. Give the tank a fortnight of stability first.

Get the tank cycled and running with nothing in it but rock and flow. Do a full round of testing to establish a baseline. Add a cleanup crew, then a fish or two, then your first soft corals.

Do not buy dosing equipment until a test tells you the tank is consuming something. That day will come, and when it does you will already know your consumption rate because you will have been measuring.

Reef chemistry is not difficult. It is just relentless, and it rewards people who write things down.