Other than time, nothing really.
However, since getting into the hobby again and liking it so much and thinking how hard it is to take care of all the horses we have, how much work it is to show them, foal them out, etc... Just looking back and seeing what an emotional roller coaster horses have sometimes been -- It's made me think that I may give up some horses in favor of this less stressful hobby. We'll see... My main hobby (horses) can be so expensive that the fish don't seem to be much of a blip in the $ department.
I'm pretty sure I will not ever be up all night with the vet out working on a fish as we've done with horses, probably will not cry over a fish or have huge guilt feelings (like loosing a mare during foaling -- a mare who was a great show horse, driving horse and pet and didn't HAVE to have foals but I wanted one out of her). Just so much. Fish are easier. Fish are more relaxing. Fish are WAY cheaper.