Thanks Byron. I will try to get or find a pic of these things. So far I have not found one on the net and they're so tiny that I must really strain to see them with the naked eye, but I will try to work something out. My tank has mostly Rainbow fish including one Austrailian, two Red-Tailed, two Praecox and one New Guinea Rainbow. I also have a large Amazonian Tetra; A Clown Pleco; a Bumblebee Cat; a Salt & Pepper Cat; three Neons and two Harlequins. I use an AquaClear 50 w/ the 3 elements; mech. filter, carbon and ammonia. I do maybe 1/3rd water changes about every month. I vacuum the gravel maybe once every 3-4 months. I test for ammonia and nitrites about once a month or so. My temp is pretty steady at 78 F. I've had very good luck with my aquariums over the years but my knowledge is limited to personal experience so any help or info is greatly appreciated.
From this information, I would suspect these are planaria or similar. They are usually harmless to fish, and reslt from overfeeding and/or insufficient cleaning of the substrate/water changes/filter. And the latter is certainly present here.
Water changes must be weekly at a minimum, once each week. Change 1/3 to 1/2 the tank volume. Vacuum into the substrate especially in open areas such as along the front. If you have live plants rooted in the substrate, don't vacuum around them, just the open areas.
Tank size isn't mentioned, so I can't say if the fish load is or isn't an issue combined with the above. Pleco are notable waste factories, though the Clown is small (3-4 inches) but without more regular water changes/substrate cleaning this can still add up.
Planaria are small, you may need a magnifying glass to see them more clearly on the glass to ascertain their shape. I've sometimes seen very tiny white worms, but not more than a very few, and in my 10g which is loaded with dry leaves decomposing [my Farlowella fry feed on these].