The tank in which I saw this product being used contained Sulawesi snails, shrimp and Khuli loach. The whole tank was treated with the sick fish (moonlight gourami) in place. No special hospital set up. There were no losses, the instructions were followed precisely.
Hope that reassures you. In any case you can add half dose and observe, if there seems to be no issues add the remainder. That's how I deal with dosing my scaleless fish (loaches/puffer)
If you need to, remove half the water, dose tank at 50% by volume, then if the fish does seem stressed you can quickly dilute your medicine dose.