Every reefer eventually collects a shelf of test kits and a spreadsheet, then discovers that the tank still is not happy. The problem is usually not which numbers you are hitting. It is which numbers you are chasing, and how hard you are chasing them.
The five that matter most
If you only track a handful, track these.
- Salinity — 1.025–1.026 specific gravity, or about 35 ppt. Use a refractometer, calibrate it, and remember that top-off is what actually controls this. Most “mystery” problems in new tanks are a swinging salinity caused by evaporation nobody is replacing consistently.
- Temperature — anywhere in the 76–80 °F range is fine. Stability inside that range matters far more than the exact number. A rock-solid 79 beats a 77-to-81 daily swing every time.
- Alkalinity — 8–9 dKH suits most mixed reefs. This is the parameter that will punish you fastest if it moves. Corals tolerate a slightly-off value they have adapted to; they do not tolerate a value that jumps around.
- Calcium — 400–450 ppm. Calcium moves slowly and rarely causes acute trouble on its own. Test it, but do not obsess over it.
- Magnesium — 1250–1350 ppm. Its main job is keeping calcium and alkalinity in solution, so if those two will not behave, check magnesium before you troubleshoot anything else.
Nitrate and phosphate are not the enemy
This is the most expensive misconception in the hobby. A generation of reefers was taught to drive nutrients to zero, and then wondered why their SPS went pale, their zoas would not open, and dinoflagellates took over.
Corals need nutrients. Sensible targets are roughly 2–10 ppm nitrate and 0.03–0.10 ppm phosphate. A tank reading a true zero on both is not clean, it is starving, and starving tanks are the ones that get taken over by the organisms that can live on nothing.
What actually matters is the ratio and the stability, not the smallest possible number. If nitrate is climbing and phosphate is bottomed out, or the reverse, that imbalance will show up as algae or as pale corals long before either number looks alarming on its own.
The three rules that beat any target list
1. Stability beats perfection. A tank held steady at 7.8 dKH will out-grow a tank bounced between 7 and 9 in pursuit of 8.5. Pick a value you can hold and hold it.
2. Change one thing at a time. When something is wrong, the temptation is to fix everything at once. Then you cannot tell what helped, and if the tank gets worse you cannot tell what hurt. One change, then wait.
3. Test the test. Kits expire, refractometers drift, and a dodgy reading has caused more crashes than any actual parameter. If a number surprises you, retest before you dose. “Emergency” corrections based on a bad reading are a leading cause of real emergencies.
How often to test
For a new tank: alkalinity and salinity a couple of times a week, everything else weekly, and write it down. For an established, stable tank with a consistent dosing routine: alkalinity weekly, the rest every two to four weeks. The point of a log is not the individual readings, it is the trend. A number in range but moving steadily in one direction is information; a single number in isolation usually is not.
Keep the numbers where you can see them
Get our free reef water chemistry quick-reference chart — every target range on one page, no charge, print it and tape it inside the stand door.
If you want the full picture — what each parameter does, what a bad reading actually means, how to correct safely, and how to spot testing error — the Reef Water Parameters Quick Reference Guide is an instant PDF download. It is also part of the Complete Reef Care Library.