fixing

I've got a tank that has a leak at one corner, near the top. If I drop the water level two inches, the water stops coming out. I know that you're not supposed to try to seal only part of a tank, because new silicone won't adhere to old silicone, but I am sorely tempted to remove the upper...

Anyone have any experience fixing fluorescent fixtures? I have 2 48" double T8 fixtures (I think that they're T8 - definitely not T5 and probably too new to be T12) where only one of the two slots/sockets(?) for bulbs works. You can get the other one to turn on by taking the functioning one...