Hi Clint
I have used it in a 5.5 nano. I used the recomended dose, and them ran a
canister filter filled with carbon for about 4 hours after. *Not 1 Loss*----The key is the carbon/filter. The
flatworm exit itself is safe---its the toxins that come out of the dead flatworms that will wipe your tank out. You will be fine---Just make sure you run plenty of carbon after you dose, and follow up with a water change....Best of Luck!------Anthony
PS: Suck up all the dead worms you can as you see them dying, this will limit the toxin output.