its happened to me and my gf, we got along pretty good for awhile then she told me that i was being too immature and freakish for keeping fish and said she wouldnt date me if i kept them, but i hardly talked about them, sometimes things like this happen, id recommend just sitting down and telling him how you feel about it all and maybe give a threat or 2 just to see how he reacts, but nothing extreme ya know?
lol. but seriously look at this as a plus your both doing something you love. next time he wants to get a fish let him know you want to spend more time with him and suggest going to the movies or bowling instead of a fish store. make it fun. have some competitions if you win in bowling he has to get you a fish if you lose youll get him one. im a guy and i dont catch hints from my girl dont get frustrated be straight foward