Reply To: Small, thin worms in breeding tank.
Flat Worms
One common critter is a tiny white “worm” that seems to glide around on the water surface and sides of the tank. These organisms are Planaria (commonly called flat worms). Some species of flat worm can be very tiny — almost microscopic. All Planaria glide on cilia and slide (some say like a snail or slug) along surfaces such as the glass sides. They can glide on the glass front of your tank and do in fact glide across the underneath surface of the water using the water’s surface tension. Planaria are sometimes incorrectly confused with microworms. (Microworms are an excellent food for fish fry).
The distinction between planaria and microworms is easy to see. Although microworms and the small planaria are similar in size, microworms cannot move along a flat surface under water. They flex from side to side and cannot swim or glide on a surface that is submerged.
Planaria in general are harmless, usually appearing in large numbers only when large amounts of food are present. In aquariums, food typically consists of excess fish food due to over-feeding. There are several freshwater and marine forms. Some of the marine forms can get quite large. Larger freshwater species are themselves eaten, in some cases, by certain types of fish. One of the most common of the larger freshwater species (and used frequently in Biology classes) is Dugesia. Dugesia can reach a length of slightly over a centimeter long and is further characterized by having an arrow shaped head and a grayish color, not white.
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Turbellaria
Order: Order Tricladida
This is it i have it in some off my aquariums where i overfeed nothing to be afraid off they are harmles, you can reed more here
http://members.aol.com/larval1/critters.htm