Flatworm eXit itself is quite safe to 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 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.
Important remarks, Below given remarks are a.o. meant to reduce the amount of the flatworm’s toxic body fluids in the water. Many people have treated their reef tank(s) successfully using Flatworm eXit without any problems. The most important factors are reducing the number of flatworms before commencing with the treatment and reducing the amount of the flatworm’s body fluids, released when they die, as fast as possible. Do not underestimate the number of flatworms in your system. In a few cases a significant number of flatworms can be hiding in for example crevices. Blow in such and any other potential hiding places water to blow any flatworm out of there. For that purpose you can use a powerhead or a turkey baster. Siphon out those flatworms as well. Furthermore do not underestimate the amount of toxic body fluids these flatworms can excrete. This can be reduced significantly by really siphoning out the flatworms before commencing with the treatment. Further significant reduction is accomplished by siphoning out as many dead flatworms as you can.
Any body fluids released by the dead flatworms in the water has to be reduced further by using about 1 pound of fresh high quality carbon for every 50 gallons. The carbon has to be used in a canister with a sufficient forced water flow. Take sufficient time for treating the tank. That is, do not treat your tank in a hurry. Also take time to monitor your tank for at least 6 hours after the treatment. It is also wise to have at least 25% water ready for a water change if required.