Raise the temp on the tank to about 82-84F and add some salt. Are there more fish in the tank (I hope not, as six tetras would be enough in a 10gal)? When I used one of my 10gals as a hospital tank for my female bettas with ick, I tore it down completely, scrubbed it out with a vinegar and water solution, and boiled the gravel and everything "loose" in the tank (thermometers, heater, sponge filter, etc). Haven't had a problem since and I was using that tank as a temporary home for 4 male guppies, 5 platies, 3 otos and an angelfish until last night.