My 4 month old dumbo betta developed signficant fin shredding in the tail and anal fins. He always had an appetite, but was hanging behind the heater much of the time. I did the recommended cycle of Triple Sulfa, and he improved, however a month later, he seemed to get worse, so I used Erythromycin. Again, he became more active, and his fins looked healthier. Now, 2 weeks later, even his pectoral fins are showing missing fins. I am beside myself. He is the sole resident in a 5 gallon heated (82) tank with a very mild filter (that I took from a 2 gallon tank). I change his water (30%) once a week. There's no ammonia, and nitrates are 0. Should I do another round of erythromycin? How many times do you medicate fish before you stop? If I stop, surely, he will not survive.