There are very many different kinds of marine flatworms (Platyhelminthes) — how did you ever manage to identify the exact species that have appeared in your aquarium? That’s some good detective work!
There are a few parasitic flatworms that plague seahorses, such as tapeworms (Cestodes) and blood-sucking flukes or trematodes, which are like tiny leeches, but these require a host to survive. Most of these flatworms that find their way into our aquariums are free-living (nonparasitic), and the vast majority of these are benign, harmless creatures that are best ignored, although they can be troublesome when their population gets out of control. Here is a link to a good article on flatworms by Bob Fenner, which is loaded with good information and a number of pictures, and may help you to determine if your flatworms might present a risk to your seahorses:
Click here: Flatworms
If you want to be absolutely safe, Barbara, you can certainly eradicate the flatworms from your tank. Flatworms are more of a risk to reef keepers because some species attack clams and certain types of corals, and reefers swear by a product made by Salifert called Flatworm Exit. It wipes out flatworms but is said to be completely harmless to other invertebrates and fish when used as directed:
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. 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.
The following remarks are 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.
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.
Flatworm Exit is available online from a number of different sources. I have never used this particular product, Barbara, so I cannot vouch for it or say for certain that it’s safe with seahorses, but if it’s used in reef tanks and doesn’t harm the delicate corals and invertebrates, it’s very unlikely it could be harmful to seahorses when used as directed.
If you find the flatworms unsightly, or feel they may present a health risk to your seahorses, go ahead and eliminate them. Just observe the safety precautions outlined above.
Best best of luck getting rid of those pesky flatworms, Barbara!