I bought Maracyn to treat what appeared to be a bacterial infection killing my neon tetras (that I had purchased at a big box store - a mistake). I lost a few tetras before the Maracyn could start working, but it stopped the infection in its tracks. All the other tetras are happy & healthy. It also didn’t harm my snails, shrimp, or plants.
If you have a 30 gallon or larger tank, you’ll definitely need the larger pack.