Hi,
I have several tanks with koi in them. The other day I was changing the water when I discovered what looked like tiny brown worms in the filters, and upon closer inspection, I found them on the glass. :o What are they, do I need to worry about them, if so, how do I get rid of them? :(
There have been no injuries, or illness, or new fish in the tanks. Water condition has been optimal.
Any ideas?

Thank you.

If they were on the glass and in the filter, they are probably not parasites so you don't have to worry about that. If they have arrow-shaped heads, they are planaria which are flatworms. Planaria are the most common worm to be found climbing glass. There are many kinds of worms that may end up in an aquarium including those commonly sold as food such as blackworms and tubifex worms. Most of these types of worms feed on leftover food in the tank. Simply cleaning the tank more often may be all that is needed to knock back their population. I have a section on planaria at http://www.fishpondinfo.com/micro.htm

Thank you for the information. After looking at your page on worms, I'm pretty sure that it is the Planaria. It's nice to know that it's nothing to worry about. It will definately put my daughter's mind at ease! :D

In your info on Planaria, you mention getting paradise fish, or something similar. Since koi get large enough to eat these fish, what would you recommend I consider using? I have a plecos in each tank, and some corey cats, but I have to provide the cats with a "safe haven" or the koi will eat them. I also have an indoor pond, do I need to have some in there too?

Since the planaria are not directly harmful, I wouldn't worry about adding fish to eat them. When I had planaria, it was usually an explosion of them that lasted weeks but then stopped. I haven't had the white ones in years. When I vacuum my tanks now, I get a few larger black ones in my sucked out water. They're kind of neat actually. To keep them under control, simply do good tank maintenance. They have to eat something. If it's clean, they starve.

P.S. Keep an eye out that the koi aren't bothered by the plecos or cories. Koi grow big fast so I'm sure it's hard to keep their tanks clean. They really need to be in ponds unless they're still tiny. Good luck!