Fin Rot.. What do I do?

Jan 31, 2009
81
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Near LA
#1
Shortly after the sick Red Wag platy died of unknown causes, I noticed frayed fins on one of the females. Her top fin is ragged, and half of one of her bottom fins is gone. The RW had a frayed top fin, which I was told had been caused by him running into the filter intake. Now I'm pretty sure it was fin rot that may have turned into something like gill disease which I couldn't see. Now that the female is sick, I want to treat it right this time.
Questions:
1. Is fin rot contageus? Is my whole tank going to get it?
2. Should she be in a hospital tank? I have a spare 1g sitting on the counter with some gravel (and hopefully good bacteria) from another tank. It has a mini heater (for 2-5g tanks-is this a problem?) and a sponge filter.
3. What medicine do I use? I have some that turns the water red that I used for mollies with popeye, but it really messed up my tank. (macaryn-tc by mardel)

Thanks everyone!