Sickness or fish fighting?

[Jonatheber]

I have a fairly new (about two weeks) tank with water that has no ammonia, nitrate, etc. I had a few fish die right after I stocked it, so I watched fairly closely and saw a guppy acting very aggressive (biting at the side of one of the fish). I disposed of that guppy, and added 10 new fish - some guppies, two Mickey Mouse plattys and some neon tetras. Overnight, all the tetras were gone, but the guppies were fine. I suspect the tetras died because they were newly arrived that day at the LFS, and moving them twice in a day probably stressed them out. Two days later, I noticed have a few fish that have an odd appearance to their tails or fins. Two new plattys seem to have chunks missing - 1 from its tail, and another from its top fin, and one of the three painted glass tetras I had since the beginning has some white stuff on its fins and tails. It has also basically been hanging out in one of the corners for most of the day. I have see NO signs of any fish chasing after any other fish. The biggest other fish I have are three Dalmation Mollys, and they basically seem mostly interested in picking at the gravel to eat. The other two painted tetras seem perfectly fine - no white stuff, and they are swimming all over

Do I have another fish picking on the others? Or is the white stuff on the painted glass tetra a sign I have some sort of disease. I posted pictures (including two of the tetra) on this listing.

 

[Delapool]

Sounds like fin rot - possible from either lfs / new tank. Here tetras really need an established, cycled tank.