Epoxy is not silicone at all. It's totally different, but some brands are known to be safe for tanks and livestock. The stuff sold in aquarium stores is very expensive, but safe.
Oatey's is brand sold for use by plumbers on water pipes. I buy it at Home Depot. It is safe also, I know several people who have used it and had no problems as well as myself. It's just fast curing so you only get a few minutes to assemble after it is mixed, and is a pale colour.
You can also use tank safe silicone.. if it is tank safe, it's fine. GE makes one, I forget if it is type I or type II that is ok.. but if it says anything about preventing mould or mildew, it's NOT ok. Only silicone, not acrylic or any other type, and if it says it is for bathrooms and kitchens, it's not the right kind. They all have anti fungal stuff in them that's not safe.
There are some other glues that are safe too. The one thing they all share is price, it's fairly high, and I am not sure what brands are available in any given place. But Oatey's is, I think, American made.
Silicone is likely the least expensive, but you can end up with a lot left over and it does not keep, even if tightly closed, it will cure and have to be tossed out. The epoxy putty lasts a long, long time kept well wrapped so you can use it again later.