Here's something you might want to try:
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http://www.syngnathid.org/ubbthreads...b=5&o=&fpart=1
A Treatment for Hydroids in the Tank
Panacur is a trade name, the active ingredient is fenbendazole.
This is a deworming agent and a prescription so you will probably need a vet's cooperation, which is not difficult. I purchased mine online and it is very inexpensive. I got a 4 GM pack for a couple US dollars.
Treatment is 1-2 mg per gal. As far as safety, I am not 100% certain at the present. If you run an independent system I would use that first. As far as my biofiltration, my cycle remained active and none of my seahorses had any difficulty returning to the tank 4 days later. I did not run any extra filtration.
This also killed off green algae that was in my tank and also a couple of polyps that I test treated with. What I did with this large amount was just pull a little amount out with a toothpick, point being I was unable to determine exact dosage I used, but it did not appear to matter and results were very pleasing.
I did remove seahorses for 4 days, but I did not run carbon and they did not suffer any problems. Hope this is helpful.
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Hope this helps.
Robert
Note: I have edited this post to correct the URL, which was mangled during migration from phpbb to vbulletin. The link should now be correct, for anyone who is referencing this thread. -Delphinus