the heating part seems complicated to understand, this is my attempt at it maybe u can merge with ur existing one somehow
at least 1 watt per litre, either with a single heater or a combinations of heaters adding to at least 1 watt per litre. if ur using two heaters have one set at a degrees lower than the other to prevent both heaters heating at the same time which could overcompensate resulting in a constant rise and fall in temperature.