i have two small golden tench they are about 3 inches long i am keeping them in a 4 foot fish tank indoors and hope to put them in the pond next year . i have noticed some small white worm like creatures on the glass they are about a half an inch to 1 inch long . please could you tell me what they are and could they harm the fish. i live in the u.k. thanks ken.

Ken Winters

FIRST: I do not know about treating Tench. They don't have 'em here in North America.

Numerous small white worms writhing in the water and or on the glass are NOT pathological. I'll tell you that first, so you can relax. They're jsut unsightly and they tell you something about the cleanliness of the tank. (Tank with big eaters in it, right?)

They're a saprophytic nematode or worm, that simply indicate that uneaten food is being allowed to accumulate in the system, either in the gravel, or in other out-of-the-way places.
The best treatment is a good tank overhaul and cleaning!
Treatments otherwise usually would involve Formalin, Potassium permanganate, Copper, or an organophosphate. None of these is particulary safe or "lightly" entered into.
Better than that, I would recommend getting a small goldfish or gourami, as both species relish eating these worms. Once the worms are down to reasonable numbers, you can move the goldfish to a bowl, or whatever.
The worms are most often seen in Newt tanks, Oscar tanks, or crab/intertidal systems where animals live that eat "chunks" of meat. In tropical community tanks the worms are rare, because as earlier stated, the smaller fish like to munch on the worms, which is of no detriment to them at all. (The fish that is! )

If you cannot use a scavenger fish, then simply siphoning off some water every week, along with a siphon-cleaning of the gravel will help immeasurably to reduce the worm burden in a tank.

I do NOT advocate the use of caustic formalin compounds, or dangerous copper compounds for these worms.