40 gallon planted tank been up 8 months only has 6 corys. I added 5 white skirt tetra, one died in about 48 hours figure it didn't do well with the transfer. Of the other four, three hung together the last one kept to him self hiding and I quickly noticed he wasnt eatting. After almost to weeks he was looking thin, and one afternoon found him stuck to the filter intake. Now down to three of the tetras two are hanging out and once again one is keeping separate and hiding once again not seeming to eat. Any thoughts on what caused this one to take the others place? I dont want to keep having one a hungry stike and dieing.