Yes, you’re 100% correct. You’ve described kin selection. So if grandparent fish can somehow help their grandchildren procreate more, then that can be selected for.

I’ve heard of territory defense helping with fish, but no idea about the fish mentioned specifically.

Also to the previous comment, I think there’s already a fair bit of evidence that older larger fish do continue to breed, (hence throwing back the bigger fish breeders).

But if they stop breeding, and there is no kin selection; then yes, natural selection wouldn’t play a role.