Zoanthids tolerate a wide range of conditions, but stable salinity, temperature, and alkalinity matter more than chasing perfect numbers.
Zoanthids are among the hardiest corals in the hobby, and they often do well in tanks that would stress more demanding corals. Swings are the bigger problem. A colony that sat happily at 78°F (26°C) for months can react badly to a fast drop or rise even if both readings are "in range."
Target ranges
These ranges suit most mixed reefs that keep zoanthids:
- Temperature: 75-80°F (24-27°C)
- Salinity: 1.024-1.026 specific gravity (about 32-35 ppt)
- pH: 7.9-8.4
- Alkalinity: 7-11 dKH, held steady within about 1 dKH day to day
- Calcium: 380-450 ppm
- Magnesium: 1,250-1,400 ppm
- Nitrate: roughly 2-20 ppm
- Phosphate: roughly 0.03-0.10 ppm
Zoanthids don't build a stony skeleton, so they use far less calcium and alkalinity than SPS corals. If your tank is mostly zoas and soft corals, weekly water changes may cover most of the demand.
Nutrients are not the enemy
Zoas usually do better in slightly "dirty" water than in ultra-low-nutrient systems. If nitrate and phosphate both read zero for a long time, colonies can pale, shrink, and stop growing. Some keepers chase zero nitrate for stony corals and then wonder why their zoas are fading. Aim for low, measurable levels rather than none.
On the other hand, very high nutrients feed algae and cyanobacteria, which smother polyps. If you see nuisance algae creeping onto colonies, bring nutrients down gradually and improve flow at the same time.
Testing routine
- Check salinity and temperature every few days, more often in hot weather.
- Test alkalinity at least weekly, and more often if you dose.
- Test calcium, magnesium, nitrate, and phosphate every one to two weeks.
- Log results so you can spot trends before the corals do.
A refractometer calibrated with a proper reference solution (not RO water alone) gives far more reliable salinity readings than a swing-arm hydrometer.
Making changes safely
- Change one parameter at a time.
- Correct alkalinity by no more than about 1 dKH per day.
- Adjust salinity slowly, by about 0.001 specific gravity per day.
- Top up evaporation with fresh RO/DI water, not saltwater, or salinity will creep up.
Stability habits that help
An auto top-off, a reliable heater sized for the tank (or two smaller heaters for redundancy), and a temperature controller do more for zoanthid health than any additive. Watch summer temperatures in particular. Keep the tank out of direct sun, and consider a fan or chiller if your room regularly pushes the water above 82°F (28°C).