I think my goldfish might have fin-rot.

Question
I recently set up a fish tank about two and a half weeks ago. It's a ten gallon tank with four neon tetras, a platy, and a young common gold fish (he's a little less than an inch in length right now). I found him originally by accident at a festival dying in a sink in the bathroom with less water than he could swim in, and decided to save him and take him home (which is why I ended up setting up the tank in the first place). I have a filter, an aerator, gravel, a few medium sized rocks, and a place for my fish to hide when nervous. The water in the tank is un-chlorinated natural spring water. Unfortunately, because the tank is in my room, they don't have normal night/day cycles, although I do make an attempt to keep them on a schedule and try to turn the lights off around nine pm. I feed them tetra-fin goldfish flakes twice a day (I am considering switching to more inclusive pellets though), once around 7am and again around 5pm. I know my tank is going through the cycling period so I'm afraid to do any water changes, but my goldfish seems to have developed what I think might be fin rot. He's got a small patch on the edge of his tail-fin that looks white and dark-silver-grayish. I think it might be slightly frayed too, but he's too active for me to get a good enough look. It's not very bad yet (and doesn't seem to be affecting his behavior or ability to swim), but I'd like to know what would be the safest way to treat it without messing up the cycling process? Or should I give it a few days to see if it will go away on its own before doing anything? I've looked at my other fish, and the goldfish is the only one with this problem. (They are all highly active and seem to be quite healthy) Will it spread to my other fish? If so how can I stop that from happening?  

Answer
Hi Em,

I realize you're doing the best you can for the goldfish, but as it grows it's going to make life difficult for you and for its tankmates. Common goldfish grow very large, they are really pond fish...this is not an uncommon size for a comet:
http://www.carp-fishingholidayfrance.co.uk/images/SSCN0610.JPG

Fortunately, the tetras and platy will fit just fine in your 10 gallon tank, so as soon as you can rehome the goldfish, your tank should stabilize. The longer you keep the goldfish in there, though, the more dirty, cloudy and unmanageable your tank will be prone to being. So you really must get in contact with someone ASAP (preferably with a pond) who is willing to take this fish...

Unfortunately, even a small, juvenile common goldfish will produce a lot of ammonia, more ammonia than a 10 gallon tank's filter can handle. That's because goldfish of any sort, produce much more ammonia for their size than do most fish...their stomachs do not process food the same way as fish like tetras and platies. Goldfish excrete "raw waste" - and lots of it! Your 10 gallon tank will never look clean or clear if you have a goldfish in it.

Cutting back on feeding (feeding only once, a small portion, every other day) will help to add less nutrient load to the tank. You should see an immediate improvement after switching to this schedule.

I would add some aquarium salt (you can also use kosher salt, or non-iodized salt, sold for cooking) to the tank. 2 tablespoons for your sized tank, for starters. In another couple of days, add this same dose again. This will help your fish deal with the ammonia and nitrite. Dissolve the salt in warm water before adding it to the tank.

Keep the tank on the cooler side, about 74 degrees should be ideal. This will keep the bacteria responsible from finrot from spreading too quickly. Bacteria reproduces much more rapidly in warmer water. Fortunately, all your fish can tolerate this temperature just fine.

If this doesn't keep it at bay, you may have to try some all purpose medication such as Lifeguard. Jungle Labs used to make it, now I think that Tetra does. It is available at Petsmart, if not Walmart (but not Petco, who no longer sells antibiotics). Melafix is very iffy, expensive for what it is (diluted tea tree oil). Lifeguard is a real antibiotic, consistently effective. If you can't find anything else, Melafix will have to do! However I recommend Lifeguard first.

I hope that helps, take care.
Nicole