I thought about this route, but I like having the temperature control integral to my controller. I also don't like that the off-the-shelf controllers have a single setpoint and on-off control. Means the heaters cycle on and off continuously. I have some hysteresis programmed into my controller. It controls the temperature between 76-78°F. This way, once the tank is heat up to 78, the heaters shut off and don't come back on till it hits 76. Reduces the cycles on the relays built into the heaters and the relays I use to control them with.