We have chatted many times about many aquarium things, and all your advice has been quite helpful. Now, another problem, - flatworms, and lots of them on several corals. Please help once again!
Yes, we have chatted numerous times, but the record is 127 emails with a reader in Cape Town, South Africa, so you have many to go before we get near that amount!
As for flatworms, there are numerous different species of parasitic ‘planaria’ flatworms. Yet, the ones I've seen the most are on soft corals and look like brown freckles with a forked tail and slightly rounded top. They are about the size of a freckle (2 - 5 mm) and can rapidly increase in number to the point where they may cover the entire coral.
Mushroom and leather corals seem to be a favorite gathering place. These flatworms are thought to be mostly harmless (Sprung, 2000), yet unsightly. When in great numbers they can block light from reaching photosynthetic animals. Some flatworms will consume both tissue and zooxanthellae and the corals may exhibit thin lines of dead tissue. Yet, have not heard of corals damaged to the point where they can’t return to full health, as most seem to slowly get better when the infestation is cleared up.
When there is an abundance of these flatworms, and if possible, remove the infected specimens and give each a five to ten second freshwater dip. Almost all the flatworms will drop off and any remaining can be gently brushed off. Of course, not all corals can withstand a freshwater dip, but leather and mushrooms do handle it quite well.
Keep in mind these flatworms are photosynthetic and are attracted to light. Placing a lamp near the aquarium side panel at night will attract them in huge numbers making it easy to siphon out the majority very early in the morning. The reduced number may give other biological control methods a better chance at bringing their numbers under control.
As for a natural biological approach, the Palauan Banded Goby, Amblygobius phalaena, is said to eat the flatworm Convolutriloba retrogemma. Also, Wrasses, e.g., the Sixline Wrasse, Pseudocheilinus hexataenia, and the Mandarin fish (Synchiropus splendidus) have cleared up the problem in some of my past reef systems. Also, the Sea Slug/Nudibranch ‘Chelidonura varians,’ - black with two blue lines on the sides and two tail-like projections on the rear - is an efficient consumer of this pest. Also the Scooter Blenny, various Hawkfish, and Leopard Wrasse (Macropharyngodon meleagris) may also consume them.
There is also some thought that the over use of iodine supplements, which flatworms concentrate in their internal fluids, may lend itself to their proliferation.
And caution, as there is some thought that killing them in large numbers with a chemical treatment of some kind could present a serious environmental hazard in the aquarium for all inhabitants. That’s because their degradation could cause a large reduction of dissolved oxygen and/or since they are thought to harbor iodine in their tissues, the system may become overdosed.
Using the light/siphon out method has worked for the majority of aquarists having this problem. And having a few predators of these worms in the aquarium is a good back-up!
Hope this helps,