Tetras are more sensitive to medications than other fish.

On most medications, there are two dosages, one for most community fish, and a lesser dose for tetras. If you have tetras mixed with other fish, as most of us do, it's somewhat of a lose/lose situation ... you either over-medicate the tetras and the medication kills them, or you under-medicate the other fish, and the disease kills them.

Sorry about your fish.