Since your fry are doing well, it "ain't broke." Don't fix it.

Stuff found on the beach usually should be soaked. Use freshwater for marine shells, saltwater for freshwater shells. I always wonder what/who is still in there.

Better to soak than be sorry. This is where getting a couple of fairly inexpensive buckets (an "In" and an "Out" bucket) just to be used for the fish is good. I would bleach (outside in the shade) in the Out bucket so it gets cleaned up too.

If you have a bunch of aquaria, it may be worth your while to buy a couple rubbermaid 32-gallon trash cans for the purpose. I still use them outside, whenever the weather allows. (I'm gambling that no freeze at this point will freeze them very solid.) Tops keep the bleach in one and the sodium thiosulfate (the dechlorinator in all dechlorinators and in the photographer's hypo solution) from dissipating into the atmosphere. Most dirt, critters and kids can be kept out too.

After a couple summers, I feel safe storing water or raising Daphnia or fish in them. If they release plasticizers, I hoping (knock on wood) that those have leached out.