A long time ago (in a galaxy far, far away
,) when I was messing around with papier mache, one recipe I ran across said to add a few drops of wintergreen oil to the paper mash to keep the mixture from molding. Found some at a pharmacy, did so, and it did indeed keep it fresh.
The problem is I don't know
what wintergreen oil would do to an aquarium.
At least the fish would have fresh breath
. . .