Worms in your aquarium. The worms in your aquarium are most likely to be one or both of the following. (a) nematodes, which appear as thin thread like white worms, that swim in a wiggling s-shape in the water when disturbed, they may also be found on the glass and surfaces and gravel of the aquarium (b) planaria, which are a short flat worm found crawling on the glass and other areas of the tank. Both types of worm are a non parasitic worm which are harmless to adult fish and there is no evidence that they pose any threat to humans. Although they could attack unhatched fish eggs but are harmless once fry are free swimming and even provide a source of food for the young fish. They can be found in both fresh and saltwater aquariums. They vary in size from microscopic to a few millimeters. Nematodes and planaria thrive in a situation where there is an excess of uneaten food accumulating in the gravel bed of the tank, typically in tanks with messy eaters. They are feeding on the resulting bacteria being produced by the decaying food. Some evidence suggests that the worms are introduced via live food and could possibly even survive in freeze dried preparations. They can also be found within the filter media, and so they can be introduced into another aquarium if media from the infested tank is used to seed another tank. They reproduce and multiply very quickly and their rate of reproduction increases as water temperature increases. Medications and chemical treatments are not necessary and not a desirable method for getting rid of nematodes and planaria. The best method of riding the aquarium of these unsightly creatures is increased gravel vacuuming and cutting down on feeding, as their food source decreases so too do the worms. They will eventually die off although it may take some time and vigilance, feeding less and vacuuming the gravel will decrease their numbers. Fig 1; outline drawing of a nematode worm. Actual size 5mm Fig 2; outline drawing of a planaria worm. Actual size 3mm It must be remembered that these worms exist in all natural ecosystems, and there is nothing abnormal as such about their presence in the aquarium. They are simply the sign of too much waste collecting in the tank, and since the aquarium is a somewhat unnatural environment and they quickly reproduce then it is preferable to eradicate them.