Tetracycline causing serious foam????

[taebaker]

I have a little black fin tetra that has severe belly swelling.
Was told at my LFS that it may be bacterial and to try tetracycline.
I put the fish in my 15 gallon hospital tank and added the prescribed amount of T/C, after removing the activated charcoal.
Went back to check on the tank an hour or so later and the surface was covered with (I mean 2 inches thick) with foam.
Is it normal for T/C to cause foam like this?????

[AngelMom]

I've never seen it do that, but I try not to use it (it's been YEARS). Could be that one of the additives is acting as a surfactant ... or is it possible there was a little something already in the tank it's reacting with?