Well standard Aquaone Heaters come in 25 watt increments: The Amount of heating required is a Watt per Litre, so you would need either a 50(may keep the temperature a bit low due to being at the limit) or a 75W heater. My choice would be go for the 75W, not that much difference in price and assurance that you can keep your temperature up. Larger heaters i find to be more reliable.
Hope that helps