For yr tank, 1/5 or 1/4 is more than enough. For drop - in, personally I feel wise for 4 ft and above unless u don't want the heat fr standalone chiller. Super overpower is definitely overkill. For ext. Thermostat, u need the controller with probe and hook to power socket. Yr chiller is supposed to set temperature low or lowest to ensure the ext thermostat once reached the required temp, will get activate power supply to yr chiller to kick start and run. One ideal temp reached, it will cut off power supply to yr chiller. For artica, may not require but for other brands, good to have as their internal thermostatt usually turned faulty. For drop in could, ###### seems safe but any leakage is likely to kill all yr livestock. Copper coil with rubber insulation, danger is rubber turns brittle after something and exposing the copper to rust thus leak. Best is a drop in in isolated area.