A lot of us are using duct tape to seal the top of the tank. It's OK to use it above the water line. But, I know that I've let some tape slip into the water from time to time, and nothing happened.
I replace the duct tape frequently, by keeping a big role of the tape near the tank. The pieces that hold down the moveable lid are replaced every night. The more permanent pieces stay a month or so.
It's nice that it comes in colors, too, so you can match your tank!