Hi everyone! Hope someone can help me out. I noticed tonight that the 2 male Diamond Tetras in our tanks have badly torn dorsal & pelvic fins. We have one other Diamond that is a female and shows no signs of torn fins. No other signs of illness (reddness, swelling, white/black spots, etc) are apparent. Tank mates are all normal. We have had them all in our 20 gallon tank for about 8 months. The larger of the males has always been the "alpha", but the other male has grown quite a bit since we first got him and his fins have been getting as long as the other males. From what I observed at the past few feedings and at length tonight, the female has been spending much more time hanging around the younger male lately while the older male now goes and hides in the plants by the sponge filter. The younger male seems to chase the bigger male away during feeding, so I'm wondering if this is a dominance battle between the males or if they are ill. In the several books we have and online, I haven't been able to find much more than that they are widely over-looked, peaceful community fish. So I am baffeled at this behavior. And don't have any experience in itentifying or treating ill fish. The tank, which is our first, has been up since Jan/Feb '09. We do 20% water changes every 1-2 weeks. We learned about the Nitrogen Cycle the hard way (our LFS seemed to be happy to just keep selling us more fish to replace the casualties until I did some research on my own. They also thought it would be a really good idea to have at least 3 RTS together in a 20G tank- needless to say we no longer shop there). Amonia/Nitrates/Nitrites are normal, our water is on the hard side with pH 7.4. We have an Aqueon Power 20 with an added 30G Zeolite bag, a sponge filter, and air stones. Current residents of the tank are: 2 small Otocinclus catfish, 4 neon tetras, 2 rummy nose tetras, 3 bloodfin tetras, 3 diamond tetras. The tank has been stocked this way for about 3 months now. We originally had a school of 9 neons and a red tail shark in this tank with the 3 diamonds, and 5 rummy nose in another 20g tank with the bloodfins but the neons and rummy noses kept disappearing from each tank. The otos are the newest residents being added 5 and 3 months ago (one at a time- these guys are so great! I want more!). We had problems with the RTS getting territorial and chasing the diamonds (-all the other small fish just hid in the plants), so we removed the RTS 3 months ago and added the remaining rummy noses and bloodfins to this 20G tank. The RTS is now in a 10g with 3 small tiger barbs (Rena Smartfilter 20, sponge filter & airstones). "Santa" is supposed to be bringing us a 55 gallon set-up for our tetra tank and RTS & the barbs will inherit the 20G. (10G will go back to being a QT/hospital tank). The extra 20 tank "mystoriously" got a 1inch scallop chip in one of the sides while I was at work this summer, so I am not confident in filling it for fear that it will break. Sorry for the length of this, I just wanted to provide as much information & background as possible. I want to make sure the diamonds will be healthy when we move them to the new tank in about a month. Do I need to separate the 2 male diamonds from each other. Will their fins heal? What do I treat them with? Is fin-nipping typical behavior for Diamonds? Thank you for any help! Happy holidays!