What Water Parameters Should a Reef Tank Stay Within?
Reef keeping is parameter keeping. Fish-only tanks forgive a lot, but stony corals build skeletons from the water column, so they react to every shift in alkalinity, calcium and magnesium. The keepers who succeed are the ones who test on a schedule, write the numbers down, and watch the trend rather than reacting to a single reading. One bad test is noise; a drifting line is a warning.
What are the ideal reef tank water parameters?
Target salinity 1.025 SG, temperature 76 to 78F, pH 8.1 to 8.4, alkalinity 8 to 9 dKH, calcium 400 to 450 ppm, magnesium 1250 to 1350 ppm, nitrate under 10 ppm, and phosphate 0.03 to 0.10 ppm. These are mixed-reef targets. The exact figure matters less than holding it steady week over week.
These ranges approximate natural seawater, which is what corals are adapted to. Alkalinity, calcium and magnesium work as a linked trio that drives skeletal growth, so they need to move together. Nutrients tell a different story: a little nitrate and phosphate feed coral zooxanthellae, but too much fuels nuisance algae and cyanobacteria. The art is keeping nutrients low but not stripped to zero.
Why is alkalinity the parameter that matters most?
Alkalinity buffers pH and supplies carbonate for coral skeletons, so it is consumed fastest as corals grow. A swing of even 1 to 2 dKH in a day can burn coral tips and trigger tissue recession. Most reef crashes trace back to an alkalinity spike or crash, which is why experienced keepers test it most often.
Because demand rises as a tank matures, a setup that held 8 dKH last month may be dropping by lunchtime if coral mass grew. Dosing systems and calcium reactors exist to replace what coral consumes between water changes. Whatever the method, the goal is a flat line. Reef Central and Bulk Reef Supply both stress consistency over the exact target, since corals adapt to a steady 7.5 dKH far better than a bouncing 8 to 9.
How often should you test reef tank water?
Test salinity and temperature daily or with a controller, alkalinity two to three times a week, and calcium, magnesium, nitrate and phosphate weekly. New tanks and heavily stocked SPS systems need more frequent testing. Once dosing is dialed in and the trend is flat, you can ease off, but never stop watching alkalinity.
The point of frequent testing is to learn your tank's consumption rate, not to react to every digit. Log each result with the date, then read the slope. If alkalinity falls 0.3 dKH per day, you know exactly how much to dose. A logbook turns scattered readings into a predictive tool, so you correct a drift while it is small instead of after coral bleaches.
What reef parameters should I track at a glance?
Keep this reference handy: salinity 1.025 SG, temperature 76 to 78F, pH 8.1 to 8.4, alkalinity 8 to 9 dKH, calcium 400 to 450 ppm, magnesium 1250 to 1350 ppm, nitrate under 10 ppm, phosphate 0.03 to 0.10 ppm. Treat these as bands, log readings, and act on the trend.
| Parameter | Target range | Test frequency |
|---|---|---|
| Salinity | 1.025 SG | Daily |
| Temperature | 76 to 78F | Daily |
| Alkalinity | 8 to 9 dKH | 2 to 3x weekly |
| Calcium | 400 to 450 ppm | Weekly |
| Magnesium | 1250 to 1350 ppm | Weekly |
| Nitrate | Under 10 ppm | Weekly |
| Phosphate | 0.03 to 0.10 ppm | Weekly |
Key takeaways
- Hold salinity 1.025 SG, temperature 76 to 78F, and alkalinity 8 to 9 dKH for a mixed reef.
- Keep the calcium, magnesium and alkalinity trio balanced; they drive coral skeleton growth.
- Stability beats perfect numbers; a steady value is healthier than a bouncing one.
- Alkalinity is consumed fastest and causes most reef crashes, so test it most often.
- Log results and read the trend so you correct drift while it is still small.
Frequently asked questions
- What salinity should a reef tank be?
- Most reef keepers run 1.025 to 1.026 specific gravity, equal to roughly 35 ppt, which matches natural seawater. Measure with a calibrated refractometer rather than a swing-arm hydrometer for accuracy. Keep salinity steady by topping off evaporation with fresh RO/DI water, since only freshwater leaves when water evaporates and salt stays behind.
- Is nitrate always bad in a reef tank?
- No. A small amount of nitrate, often under 10 ppm, feeds coral symbiotic algae and supports color. Zero nutrients can actually pale corals and stall growth. The problem is excess nitrate, which fuels algae and cyanobacteria. The goal is a low, stable level rather than total elimination, balanced against phosphate in a similar low range.
- Why did my coral start losing color or tissue?
- Sudden alkalinity swings are the most common trigger, especially after a large dose or water change. Light changes, rising nutrients, temperature spikes and new livestock also play a part. Check your logged alkalinity trend first for a recent jump or drop, then review temperature and nutrients. Correcting slowly is safer than another big swing.
- How much does temperature matter in a reef tank?
- A lot, but stability matters more than the exact number within 76 to 78F. Corals tolerate a steady 79F better than daily swings between 76 and 80. A heater on a controller plus a fan or chiller for warm rooms keeps the range tight. Rapid temperature spikes stress coral and raise the risk of bleaching.
Exotic Pet Husbandry Research, BigBalli. We translate aquarium care into daily, trackable numbers, cross-checked against sources including NOAA and the US EPA.
Crittora provides husbandry and educational information, not veterinary advice. Sudden livestock loss, disease outbreaks or persistent water quality problems may warrant help from an experienced aquarist or aquatic vet.