I started my aquarium three weeks ago with 2 platies and 2 neon tetras. One neon tetra didn't survive the first night so I was waiting to see if the other fish would be okay. Then I added 2 corries (I guess that's the name - my kids call them "sweeper fish" or "cleaning fish") and they are fine, too. Then I added 3 new neon tetras - I read that they should be in a group, well , my poor first one was alone for three weeks. As soon I added them to the tank (yesterday) they teamed up and swam "happy" together, all 4 of them. Today one always separates itself from the others and if they come to close it attacks them and chases them away. What's wrong? What can/should I do?