The problem with using a higher wattage than recommended is that if it fails in the on position you can very quickly cook your fish. Using a lower wattage means your heater will work harder and might increase the likelihood of it failing in the off position, however, in a heated room it is less likely that the temperature will fall to a lethal level before you spot it.
On balance I prefer to use the recommended wattage rather than adding additional capacity.
I used to love my separate heaters and external stats but couldn't find these in the UK last time I looked.