K W

I've had my betta fish for a year, and he was alright for quite a while but over the past few months has developed a rather stubborn fin rot. His fins were rotted down to almost stubs. I got some wrong information in healing him, which is why he got to this point. Then I got some actual antibiotics and put him on it for a couple of weeks, following the directions and whatnot. I bought a big tank with a filter and everything and have been keeping him well-fed. His fins started growing back. I left for two days, came back, and he's laying on the bottom of the tank, gray, gills are moving rapidly and he's hardly swimming (usually he is non-stop swimming). I think the fin rot has gotten to his insides...but I don't understand how it can come back this soon after just getting of antibiotics for two weeks, and after only being gone one day, there is such a huge change. Maybe this guy (who is extraordinarily small for a betta) is just a runt and generally not healthy and has a bad immune system.
Note: the water does seem to smell moldy. I did a 30% water change today and changed the filter.

If you have any suggestions, please let me know.