01-09-2011, 11:36 AM
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Heya, welcome to the forum!! well the answer I imagine probably won't be great for ya. A couple things to note, goldfish have a very large bioload. They are dirty dirty, so having extra filtration then you would normally for the same amount of fish is a must. Second, i think 30 liters is just under 10 gallons right? If so, this is too small for one gold fish let alone 2. It is too small in size, and too small for a bioload. Depending on what kind of goldfish it is, it requires anywhere from 30-55 gallons at a minimum. (113 -208 liters?)
Fin rot is caused by bacteria, which in result is caused by poor water quality. You are doing the right things, but unfortunately with the wrong equipment. At this stage I would recommend returning the goldfish and changing them in for something more appropriate your fish tank, or getting a bigger tank. In the mean time continue doing 50% water changes daily with a good water conditioner. Adding medication will not help much or for long if you don't fix the source of the problem.