If a single heater is capable of heating the tank above the desired temp, but not to a destrcutive point... then that single heater is the ideal size...
agreed. however if you are on vacation and that single heater fails in winter (assuming you live in a temperate climate) you may come home to a tank of dead fish. similarly the heater could fail in the ON position, and you still have dead fish.
if you had used two heaters but of a smaller wattage the scenerios are the following:
1. if one heater fails, the other will likely keep the tank warm enough not to lose any fish until you get back.
2. if one heater gets stuck on, the other will not come on, and given that it is a smaller wattage, you won't lose any fish.
so, using one heater (that failed) could result in dead fish.
using two cannot.
unless BOTH either got stuck ON or OFF at the same time, which the chances of that happening at the exact same time are almost nil.
there is just less of a chance of losing fish using two smaller heaters than one larger one.
