Based on the information you have given: 180x60x60cm tank. Water at 26 degrees. Min temp of room is 16 degrees. Also assuming 10mm polystyrene under the tank but otherwise uninsulated. When temp in room drops to 16 degrees you will need around 285W of heating to keep the tank at 26 degrees (continuous heating) So yeah I would definitely go for at least 400W to be on the safe side.