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My heater plan in any tank is 2 half rated heaters and controller.
Been doing this since starting hobby, feels foolproof, but I'm sure there are a lot of ways to shoot holes in it, but its the logic I go by.
If interested here's the plan and rationale:
Find heater requirement in watts, divide in half (say 300/2).
Purchase 2x heaters each at that half rating (150w each).
In a dual cooler/heat controller - Set one heater in the heat outlet and set its on-board controller to 80. Conect a fan or chiller to the cool outlet.
Set the controller unit itself to turn the heat on at is 78, run the set temperature at 80, and turn the cooler on if it hits over 81.
Now...
Drop the second heater directly in the tank, set at 80.
Scenarios:
Both heaters operate normal - controller monitors all, everything good.
Controller heater stuck on - the controller will shut it off, the tank heater keeps tank warm. Replace controller heater.
The tank heater stuck on - the controller will shut the controller heater off, tank stays warm. Replace tank heater.
Both heaters stuck on - the controller will shut down the controller heater, tank stays warm. Fan will decrease tank temp if required. Replace both heaters.
Controller heater fails - the tank heater keeps tank warm. Replace controller heater.
Tank heater fails - controller heater keeps tank warm, replace tank heater.
Controller fails - each heater relies upon their own temp control dial, tank remain stable. Replace controller.
Controller fails and one heater stuck on - the other heater relies on own temp dial and shuts down, tank stays warm. Replace controller and effected heater.
Both heaters stuck on, controller fails, fan/chiller fails - buy new fish
Dt temp was 72/73 per digital thermometer and glass one read 74/75.
So i moved the heater from the sump to the DT as I don't feel comfortable with that low a temp.