Thermodynamic Physics of Aquarium Temperature
Water has an exceptionally high specific heat capacity ((4.184 ext{ J/g}cdot^circ ext{C}) or (1 ext{ BTU per pound per }^circ ext{F})). Heating or cooling large volumes of water requires substantial continuous thermal energy.
The Golden Rule of Heater Redundancy (N+1)
The single most frequent equipment catastrophe in the aquarium hobby is a stuck heater thermostat. Internal bimetallic switch contacts weld closed over time from micro-arcing. If you install a single 400W heater in a 75-gallon tank and the thermostat sticks ON, it will boil your entire aquarium to 95°F within 8 hours!
Professional Solution: Split your required wattage across two smaller heaters (e.g. two 150W or 200W heaters). If one sticks on, it lacks the thermal wattage to overheat the tank alone. If one fails off, the second keeps the tank alive.
Internal Submerged Equipment Heat
Submersible AC water pumps, protein skimmers, and powerheads dump 100% of their electrical power directly into the water as heat! A heavy reef tank with 150 Watts of submersible pumps acts like a 150W heater running 24 hours a day, raising water temperatures 4°F to 8°F above ambient room temperatures.