Parameter Reference
Temperature°F
Temperature is the easiest parameter to control and the one most hobbyists get wrong by overthinking. Pick a set point in the 77–79 °F band, hold it within ±0.5 °F, and stop reading forum threads about whether 76 or 80 is "better." The reef cares about the rate of change, not the absolute integer.
Target ranges
| Reef hobbyist consensus | 76–81 °F (24.4–27.2 °C) |
| Practical set point | 78 °F ± 0.5 °F |
| Tropical natural reef (annual) | 77–86 °F (25–30 °C) |
| Cold-water danger | < 72 °F sustained |
| Heat-stress danger | > 84 °F sustained |
| Bleaching threshold | > 86 °F sustained 24h+ |
Why it matters
Metabolic rate roughly doubles for every 18 °F (10 °C) rise — the Q10 rule. That means oxygen demand by fish and bacteria climbs as the tank warms, while gas solubility falls. Hot summer afternoons combine peak demand with minimum supply, which is when tanks crash. Sustained heat above 84 °F also pushes zooxanthellae out of coral tissue (bleaching) and shifts pH downward as CO₂ becomes harder to off-gas.
On the other end, cold doesn't kill quickly but it suppresses immune response, slows feeding, and lets ich and brook chew through stock that would have shrugged them off at 78 °F.
Symptoms of drift
Too low
Below 74 °F: lethargic fish, refusal to eat, white-spot disease outbreaks within days, slowed coral polyp extension. Coral won't die fast — but ich kills the fish that feed the system, which then crashes.
Too high
Above 82 °F: gasping fish at the surface, coral polyp retraction, paling/bleaching on SPS first, dropping pH from suppressed gas exchange. Above 86 °F sustained for 24+ hours, mass bleaching is the realistic outcome.
Testing
Use two thermometers. Always. A single sensor failure is the most common cause of cooked or frozen tanks — the heater sticks on, the controller never sees the rise, and 24 hours later you have an aquarium-shaped soup pot. A second independent glass or digital thermometer placed elsewhere in the tank catches the drift before it's lethal.
Apex, Neptune, GHL, and InkBird controllers all support dual-probe setups. The cheap version: a battery digital thermometer alongside the heater's built-in probe. Either way, two readings, two power paths.
FAQ
- What set point should I pick?
- 78 °F. It's central enough that a 1 °F rise or fall won't push you near either danger zone. There is no measurable benefit to 76 vs 80 in a hobbyist tank — just pick one and hold it.
- Do I need a chiller?
- Only if your room consistently exceeds 80 °F or your lighting and pumps are dumping enough heat to push the tank above 82 °F. For most builds, a basic clip-on fan blowing across the sump surface drops tank temp 2–3 °F via evaporation — cheaper, quieter, and lower-failure than a chiller.
- How fast can I change temperature safely?
- 1 °F per hour is the conservative rule. Acclimating new livestock or recovering from a heater failure, slower is always better than fast. A 4 °F crash inside an hour kills coral; a 4 °F crash over six hours usually doesn't.
- How big a heater do I need?
- Roughly 3–5 W per gallon for a tank in a 70 °F room. Use the heater wattage calculator for the precise number based on your room temperature and target.
Sources & references
- 01Randy Holmes-Farley — Optimal Parameters for a Coral Reef Aquariumhttps://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
- 02Bulk Reef Supply — 5-Minute Saltwater Guide: Heatershttps://www.bulkreefsupply.com/content/post/5-minute-saltwater-aquarium-guide-ep12-heaters
- 03Global Seafood Alliance — Typical chemistry of full-strength seawaterhttps://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/
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