Any Help.

[naesco]

Formalin will kill everything in your tank except the fish. Run carbon, do huge water changes and check to see if your tank is going to recycle again.

When you use formalin, which you should use only in a separate tank for medicating the fish, it depletes the oxygen which is probably why your fish died.
Put a air diffuser in the tank to bring up the oxygen levels.

Good Luck