Some type of worm

[unvme41031]

ok been trying to read about flat worms and any other type of worm and I cant find a pic of one that's looks like the ones I have. The thing I have are about 1/4 inch long maybe 1/16 to 1/8 inch wide and pinkish gray. I noticed them today chilling on the surface of my tank seen about 5 or 6 not watched one swim down and burrow into sand. In my tank I have a bunch of hermit crabs a false perc 2 spider decorator crabs a green brittle star and a small horseshoe crab. In my reading about what horseshoe crabs eat I see that they eat worms that live in the sand will he eat these and should I just leave them or should I go on the offensive and do a massive clean up and try and vacuum all the sand and get as many as I can out. On a side note is it possible that these things came in on my live tubeflex worms that I feed my horseshoe crab its the only thing he likes

 

[geoj]

Convolutriloba retrogemma (rust/red-brown Planaria) flatworm

 

[auntkaren]

Those are definately red flat worms. You want to get rid of them rather quickly as they will reach plague proportions in just weeks. Themselves, alive, they are harmless to fish and inverts but they will eventually cover and kill your corals.
If you have a relative few in your tank you can safely use Worm Exit in the tank and not remove anything for safety. BUT if you have a large proportion of the flat worms in the tank, know that when the flatworms die they emit a toxin that will kill your tank!
Vacuum out as many flatworms as you can and just flush them. Then follow the instructions on the Worm Exit product package. The flatworms die right before your eyes in just seconds, they hate that stuff.
If you have a huge population of the flatworms, remove the fish, coral and inverts and put them in a temporary quarantine (24 hours-ish). Put the Worm Exit in the tank and in an hour or so, do a HUGE water change and kick your skimmer up as high as it will go. The next day you can put your fish, inverts and corals back into the tank. It seems like a huge hassle but you will be rid of those suckers.
You can get some peppermint shrimp a spotted mandarin fish or a copperband butterfly fish to catch any new ones that might be introduced into your tank in the future.
Good Luck! We had thousands upon thousands of those red flatworms in our 180 Gallon reef tank and even had to remove all the live rock and dip it too. The Worm Exit worked like a charm. We've not had one flatworm in the past 18 months and now have a CBB for insurance.