There is a moment in most new reef keepers' first year when something shifts. The tank has been running for a few months, the corals looked settled, and then the polyps stay closed. A patch of algae appears on the rock. The water looks slightly hazy. Most people start hunting for a disease or a rogue crab. The answer is usually far duller: a number drifted, and nobody noticed because nobody was testing.
Water parameters are not a box to tick once and forget. They are a habit. These are the mistakes that turn up again and again in new systems, and what to do instead.
Stability Matters More Than Perfect Numbers
New keepers tend to arrive with a list of target figures copied from a forum thread or a video, and then spend their first year chasing them. The problem is that corals are far more tolerant of a number being slightly off than they are of that number moving around.
A tank sitting at 8 dKH day in, day out will grow coral better than one swinging between 6 and 11, even though 8 is not the "perfect" figure on anyone's chart. Corals lay down skeleton from the water around them. When the supply keeps changing, they spend energy coping rather than growing. Soft corals forgive this. Stony corals, particularly Acropora and similar small-polyp species, do not.
So pick a sensible target, aim to keep your salt mix and your dosing in line with it, and stop fiddling. The goal is a flat line, not a trophy number.
Salinity: The Reading That Makes Every Other Reading Wrong
Salinity is the foundation. If it is wrong, every other measurement you take is being interpreted against the wrong baseline. Natural seawater sits around 35 ppt, which most refractometers show as roughly 1.026 specific gravity. That is where you want to live.
Three salinity mistakes worth fixing today
- Topping up with saltwater. Evaporation removes water, not salt. Replace it with fresh RO or RO/DI water only. Adding saltwater to top up slowly pushes salinity higher, and it is one of the most common causes of a tank that "mysteriously" declines over a few weeks.
- An uncalibrated refractometer. Calibrating with RO water assumes the instrument is perfectly linear across its whole range, which many budget models are not. Use a 35 ppt reference solution instead, and recheck it every few weeks.
- Trusting a mark on the sump. A line drawn on the glass tells you about water level, not salinity. Levels change with flow, filter socks and protein skimmer performance. Measure properly.
Let the sample reach room temperature before reading it, keep the prism clean, and close the cover gently. A tiny bubble under the flap will skew the result.
Alkalinity: The Number People Move Too Fast
Alkalinity is where enthusiasm causes the most damage. A keeper tests, sees a low reading, adds a dose, tests again an hour later, adds more, and by the evening the tank has jumped several points. Corals respond by sulking or, in bad cases, stripping tissue from the base upwards.
Before you dose anything, you need to know your consumption. That means testing at the same time each day for a week and writing the results down. If alkalinity falls by a steady amount each day, you now know what to replace. If it barely moves, you probably do not need to dose at all — your water changes may be covering it.
Two other traps: freshly mixed saltwater can sit at a very different alkalinity from your tank, so a large water change with a high-alkalinity salt can spike things on its own; and different salt brands vary widely, so a change of bucket can shift your baseline without you touching a dosing pump. Correct in small, gradual steps rather than one big correction, and give the tank time to respond before testing again.
Calcium and Magnesium: The Quiet Half of the Equation
Alkalinity, calcium and magnesium move together. If magnesium drifts low, holding the other two steady becomes genuinely difficult — it is the parameter people forget until nothing else will stay put. It depletes slowly, so checking it fortnightly is usually plenty.
Calcium gets overdosed far more often than it gets underdosed. If you keep soft corals and do regular water changes, calcium may never need supplementing. Test before you add.
One practical habit worth building: never dose alkalinity and calcium into the same place at the same time. They react with each other and precipitate out as a fine white cloud, which means you have paid for supplement that the coral never received — and you may find a clogged pump or a crusted impeller a week later. Dose separately, into an area of good flow.
Testing Routines: Same Kit, Same Time, Same Place
Testing only when something looks wrong is like checking the oil after the engine has seized. By the time coral behaviour changes, the parameter has often been drifting for days. Equally, testing frantically and never recording anything gives you single readings with no context.
A reading you cannot compare with last week's reading tells you very little.
Your results also depend on how you test. Watch out for:
- Expired reagents or a kit that has sat open on a shelf for a year
- Dirty or fingerprinted cuvettes, which quietly alter colorimetric readings
- Not shaking reagent bottles as instructed before use
- Testing at random times of day — alkalinity naturally shifts with the lighting cycle, so a morning sample and an evening sample are not the same measurement
- Changing test kit brands mid-week, which makes the trend unreadable
Pick one day a week, one time of day, and stick to it. Same kit, same technique, same place in the tank. Log everything — a notebook is fine, and an app is fine, as long as it is consistent and you actually look back at it.
Build a Routine You Will Actually Keep
Boring routines beat dramatic rescues every time. Something like this is easy to maintain:
- Daily: glance at the temperature, check the top-up reservoir, and spend two minutes looking at the tank properly. You will spot trouble earlier than any test kit will tell you.
- Weekly: measure salinity, alkalinity, calcium and magnesium on the same day at the same time, then nitrate and phosphate. Write it all down before you change anything.
- Monthly: calibrate the refractometer with reference solution, clean pumps and probes, and check reagent expiry dates.
One last point for small tanks. A nano holds far less water than it looks, so a teaspoon of supplement or a litre of top-up swings the numbers noticeably faster. Scale your adjustments down and make them more often rather than larger and rarer.
Get salinity right, stop treating alkalinity like a race, and keep a written record. That is most of what separates a tank that quietly matures from one that never quite settles.
Photo: Jeffry Surianto / Pexels
