I've had them in my tanks. Nearly impossible to get rid of. They multiply too fast. With a tank that small, I'd take the bichirs out, keep them in a bucket an airstone while you tear apart the tank. Throw out the substrate and clean the whole tank with bleach/water (make sure you rinse very well after) and restart. Take some seeded media from one of your home tanks so that you don't have to cycle the tank again and put your bichirs back in. Depending on the type of filter, make sure you clean that well also cause the snails can live in there too. I would toss any media from the filter OR clean it very good (it will kill all your bacteria) so make sure you seed again from an existing tank.