I've tried practically everything from the puregold website to treat a
4" ranchu for fins that keep getting fin rot after having successfully
treated in a hospital tank. My current tank setup isn't much. I have a
20 gallon bare-bottom glass tank with no tank decorations, a 6 inch
airstone, Aquaclear 300 packed with filter floss, a bag of Ehfisubstrat,
a bag of Black Diamond carbon, and then some more floss. The filter is
cycled: ammonia is zero, nitrite is zero, nitrate is 5 ppm, pH is about
8.4, GH = 250ppm, KH=6.

At one time, I use to keep this fish in a 5 gallon glass tank with just
an Aquaclear filter packed with filter floss. No carbon and no bio
media for bio-filtration. The ran the filter as a mechanical filter, so
I just changed the water every other day with tap water and plain no
frills dechlorinator. But I recall the fins on the fish were perfect
under these conditions.... never had fin rot problems.

Now, I'm wondering if the use of carbon filtration is causing the fin
rot problem. I have another goldfish in a separate tank (no
bio-filtration, no carbon, just mechanical filtration), and she has
perfect fins.

My question is are there any alternatives to using carbon that I should
try? Some other media that I could use that removes colored impurities,
keeps the water clear, etc.... Or should I just remove the carbon
from my filter and see what happens?