Pseudomugil Furcatus, Dropsy And A Revelation

I have been having a continual problem with these fish and dropsy, this morning I found another gasping at the top of the tank, none of the other fish seem affected by it.

Yesterday I visited one of the MAs that I frequent to pick up some fishy supplies and had a look at the Forktail Blue-Eye Rainbowfish there, lo and behold of the 20 or so they had in the tank about a 3rd were in the early stages of dropsy with one or two quite badly affected.  I've bought forktails there and from the other MA i visit, I cant recall seeing any ill fish at the time of purchase though. When I got to the counter I was chatting to 2 of the guys that work there and mentioned that I have a dropsy problem and I got the 'don't over feed talk'.  I said they're in a mixed tank and that they aren't particularly greedy fish, then I got the 'have you tested your water talk', my answer was yes I have and its fine AND it's only the forktails that are affected. Then i said I just took a look at yours and I couldn't help but notice that it seems you have the same problem.

One of the guys went pink and the other one then turned and said actually some fish are more prone to dropsy and other diseases usually because of inbreeding AND that he has noticed forktails are one of those fish!

My reply was 'ah well that makes me feel a bit better, I guess I'll give up trying to keep them'. I think they should have offered to take the damned things back. Would it be worth emailing customer services do you think?