I would suggest using GE Silicone II
http://reefchronicle.com/wp-content/...-reef-safe.jpg (pic), make sure it is window and door and does not have any mildue supresent in it. The window and door should not have mildue supresent anyway but it never hurts to check the back of what you buy. You can get it at HD or Rona for like 6 bucks I think. It is definatly aquarium safe and it even used to say on the back a number of years ago that it was but that was removed for legal reasons. I have used it tones and in my estimation it is the best bang for your buck and safe too. It also comes in smaller tubes that do not say window and door but it is the same thing, just make sure it is 100% silicone and you are good to go.
PS: I suggest not buying the red tube 100% silicone I as more often then not it has mildue supresent as it is kitchen & bath and it is white usually.....looks stupid.
Clear 100% silcone is the way to go......or you can get aquarium silcone for $20 a tube at the lfs......