I have a 30-gallon tank that housed 15 guppies. They all came down with fin rot, so I added some salt but that didn’t help. I turned the temperature up to 90 degrees Fahrenheit in hopes of killing the bacteria. That didn’t seem to help either and now all of my guppies except for one have died. I am afraid to add any more. Is there anything I can do to get this fungus under control?
Fungus can be a very tough thing to eradicate from the aquarium. Antibiotic remedies often damage the bacterial filter and can sometimes cause more problems than they fix. Fungal infections are typically a secondary symptom of a problem with aquarium water quality. Have you tested your nitrate or nitrite values to see if anything is raised?
Adding aquarium salt can help in preventing various diseases though it doesn’t seem to do a lot to cure a serious problem once one arises. Raising the temperature in the aquarium can work to speed up the life cycle of the ich parasite, but doesn’t typically do a whole lot for fungal or bacterial infections. In fact, many aquarists may argue that higher water temperatures not only spur the growth of bacteria, but algae as well.
A temperature of 90 degrees may be troublesome for many tropical fish species, even the common guppy. What I would recommend, if you have not already done so, is getting your temperature back to normal tropical levels around 76 degrees. Checking your water and making frequent (once weekly) water changes of 25 percent is a good way to start maintaining good water quality. Once your tank has been without a large fish population for several weeks, and if your surviving guppy stays fungus free, you may start to reintroduce some hardy tropical fish given that your water parameters stay within acceptable limits.
Fifteen guppies is a lot for one aquarium. Guppies, as I am sure you are aware, are typically very active in reproducing. I would suggest at most three breeding pairs for an aquarium the size of yours. There are several other livebearing species that are far hardier than guppies. In a 30-gallon aquarium (or if you were to upgrade to a larger tank), a single breeding pair of convict cichlids (Archocentrus nigrofasciatus) may be entertaining and would likely prove hardier than most of the common guppies offered for sale.