You can put it in the overflow (it's where I have mine on my 45g), but that gives the heater a very limited amount of water and limited amount of time to have an affect on the water temp, especially on such a larger water volumn as a 185g tank. As far as the capability of your 250watt heater, that depends on the manufacturer. They all have different tank size recommendations for their heaters. Once you get your lighting on and pumps running, the temp will come up in your tank on it's own so your heater(s) may not have to run that much. I would recommend you get another heater, probably the same wattage, and run both on your tank. That way neither has to run full power all the time and if one fails you will still have one to prevent the temp from falling much.