[Willem Fick]

If you don't have it in place already, it is a good idea to put a piece of plate glass directly onto the top of the tank, under any existing cover. It is easy enough to take off when you need to work in the tank, but will prevent virtually all evaporation, in the process also protecting your light fittings and stuff. Remember to have holes cut, through which to route tubing, as well as a small hole for feeding.