So this is my personal view… but the decision is not mine to make. Sorry, im not picking on you @ghoulishcresties but you made a lot of points iwanted to hit.
Hmmm nice idea 
dog and cats have morphs. Infact a good few morphs are named after mamals 
But really, fish isn’t that far of a reach.
But the issue is with the sites, not the breeders. These sites have very little, to no, way of filtering (pun intended) the good from the bad. As a fish guy, I genuinely think the aquarium world NEEDS a MorphMarket… more than could be measured. Its a mess to say the least.
But you will only see fish if you go to the fish category or search by “all latest”.
On the forum you can use the bell icon to mute any categories you dont want to see.
It would be as simple as creating a new “Fish~” category on the market… its a lot of work, yes. Not necessarily difficult though.
You can always ask on here in the general discussion categories. There are a few aquageezers on here.
Let me introduce @lumpy , the nicest mod to walk the internet.