Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Watts to hold temperature against a cold room, and how long a given heater takes to recover.
A heater only has to replace what the tank loses. Most of that loss is evaporation, which is why an open top tank with a skimmer and fans needs far more heater than a covered one. Size for the coldest night of the year, then use two.
watts = 75.0 gal × 5.0 W/gal = 375 W · heat up = 284 L × 4186 J/kg/K × 5.56 K ÷ 300 W
Heating water is exact physics. Water holds 4186 joules per kilogram per kelvin, so raising V litres by ΔT takes V × 4186 × ΔT joules. Divide by the heater wattage for seconds. That gives the recovery time honestly.
Holding temperature is not exact, because it depends on how fast the tank loses heat, and evaporation dominates that. An open top tank with a skimmer and cooling fans loses several times what a covered tank loses. Room humidity and airflow change it again.
So the sizing side uses the convention manufacturers publish: watts per gallon scaled to the temperature differential. Two and a half watts per gallon for a five degree Fahrenheit lift, three and a half for ten, five for fifteen. Size for the coldest the room ever gets, not its average.
Run two heaters at half the total wattage each, on separate controllers or outlets. The failure modes of a single heater are both bad. Stuck on cooks the tank, stuck off chills it, and half wattage limits the damage either way.
An external temperature controller is worth more than an expensive heater. Built in thermostats drift, and the cheap ones fail closed.
If the room is warmer than the tank target, no heater is needed and a chiller or fans are the answer instead. Evaporative cooling from a fan across the surface drops a few degrees cheaply, at the cost of much faster top off.