Couple ways to look at it... We'd be talking about a very tiny amount. Many things, most things even, are "toxic" in high enough amounts. Oxygen for example. So I'd guess you'd be safe from an amount perspective.
Second is it's an epoxy which is considered basically inert by most people. So that's a plus.
Third is the very concept of what "pond safe" means. The "test" to determine whether EPDM is safe for fish was done by Firestone and consisted of putting 6 goldfish into a small pond for 2 weeks and no fish died so "pond safe". I think most people think there were lot's of scientists involved and complex studies. Nope. Well, kind of. I mean the scientists who developed EPDM understood the chemicals involved and had no reason to think it would be toxic.
BTW, I throw out the pre-filter on pumps before installing. Properly placed in the water column even a small submersible pump should have no clogging problems. IMO the pre-filter
is the clogging problem. Pre-filters were a marketing idea imo which worked extremely well. They cost just a penny or two and they can put on the box "Filter included for crystal clear water." People who buy these pumps generally decide in the store based on packaging and who doesn't want crystal clear water? It became business suicide to not have a pre-filter.
EDIT: I forgot to actually answer your question, sorry. No one knows whether it's toxic to fish. I assume it isn't. The manufacturer says it is non-toxic to humans but suggests not eating it. Interestingly the product WaterWeld is no different from their other epoxy products or any manufacturer's epoxy product. All epoxy products as far as I know
cures underwater because it is a chemical reaction. Calling it WaterWeld on the package gets people to buy it. "Hey, these other products don't say anything about water."
