Gorilla glue (Home Depot also carries it but you may even find it in your grocery store) is the best I have used but you will never get it off the glass. Epoxy putty (I like this one best because it keeps the purple color until it is covered over but a much cheaper brand without color would suit your needs) is aquarium safe and is more likely to be removable with a razor should you want to do something different or something breaks and has to be replaced. Both break down somewhat over time (years) but are still the best I know are safe to use and have had success with.
. I never seem to remember to grab gloves but I only work with a nickel size piece at a time). Loctite makes one but there are numerous brands. As far as I know, they all cure underwater. I don't have a tube around the house of the last I bought that was not Boston Aqua Farms brand (there are other aquarium special colored brands but this one actually is a decent purple that stays purple and does not turn gray) but, unlike ammonia vs vinegar based silicones, I have not heard of any that are tank toxic.
