Unfortunately, tanks are almost always going to give you problems with humidity. Combined with a lamp, it is going to be near impossible to keep the humidity at a constant level.

Could you let us know what kind of hygrometer, thermometers you are using to measure humidity and temperature? Depending on the type of gauge you are using you might be getting inaccurate readings, which can just make this process of regulating temps and humidity very frustrating.

A picture of your setup would also help to determine what method for keeping the humidity up will work best for you.