i bought the marine/underwater version of epoxy....it comes in like this double syringe thing with two separate compounds and you just mix them together for a bit and then use it

very easy, and it even cures underwater.....it only needs to dry out of water for like 15 mins or so

plus its like 3 bucks at a hardware store....i used it to secure my airstones to the bottom of my 5gal buckets

i dont know with certainty about poisoning the reservoir but im about 2 weeks into flower right now and my plants are the healthiest ive ever grown and i dont have ph probs or anything

Hectors Hardware: Product Description

here found a link, sorry its not 3 bucks, its 4.50 lol.....o and a tip, if u do use it, use a fork or knife or something to scratch up the bottom of your airstone or whatever it is ur sticking as it greatly helps with the adhesive process
Stemis516 Reviewed by Stemis516 on . Underwater Adhesive?? help What's a good underwater adhesive, like some kind of glue I can use underwater and won't poison my reservoir? Rating: 5