Flatworm eXit itself is quite safe for fish and invertebrates. However, the body fluids of flatworms can be toxic to some reef inhabitants when present in a too high concentration. This body fluid is sometimes excreted when flatworms die. So when you use this product it is important to do the following items first: Siphon out as many flatworms as possible Keep sufficient fresh activated carbon in a canister ready. turn off UV, ozone and remove activated carbon. Keep the skimmer turned on.