Be confident in your purchase we're an Authorized Coral RX Dealer. How cool is that?!?!? Flatworms can be toxic to reef life if they are allowed to flourish in an aquarium. Once established, they can reproduce quickly and will hide in any crack or crevice that they can find. In order to make sure the solution can work effectively use a power head or other device to flush them out of the hiding places, after which it is recommended to siphon out as many as possible. The Salifert Magnesium Aquarium Water Test Kit is harmless to all reef organisms, but extreme care must be taken when using this product because of the flatworm's toxic body juice. If the toxic body juice is left unchecked in the aquarium the possible loss of aquatic inhabitants is likely. To prevent this from happening siphon out as many flatworms as possible before treating the tank. After the treatment has been added to the aquarium siphoning as many dead ones as possible will further limit the problems that could arise. Once the siphoning is finished add approximately 1 pound of fresh high quality carbon for every 50 gallons to remove any remaining amounts of toxic juices. It is also wise to have water ready to do a 25% water change if required