Cycling Issue

Normally, we are supposed to feed the fish only once a day. Feed them no more than they can finish all of it within a minute.
How often were you feeding them? From the sound of it, you were feeding them more than once a day which is not needed. More fish food means more ammonia in the system.

On a side note, Cardinal Tetra is a school fish. They need minimal 5~6 to make a school in order for them to show natural behavior.