Even if living in it 24/7 sucks for the fish, the exposure for a human that eats the fish is still transient - the tritium in the fish enters and leaves much like the water you'd drink does.

Glad to be providing helpful information. It's easy for fears to magnify and spread, humans have a bias towards paying attention to danger because that's really helpful in evolutionary terms when there could be a hungry leopard hiding in any bush.