Does this scenario sound familiar to you? You wake up one day, and go to the fishtank to look at your fish (as you always do first thing in the morning, since they’re so neat), and suddenly notice on the glass… UGH… are those..?? Small whitish worms of about a few millimeters in length each, crawling along the glass… what the heck?! Will they harm your fish? What can you do to kill them, and fast? Don’t panic. Chances are that if they are outside your fish, then these are free-living worms, since parasitic worms will spend almost all of their adult life cycle within their hosts. These particular worms don’t harm your fish.
(b) Nematode Sp.
If they are flattened and squat in body shape (but elongate as they move), then they are probably planaria (shown in a)… if they are thin, long and squirm back and forth, looking, as my friend described them “like wriggling white cat hairs”, then you probably have nematodes (b). Neither should exceed 4 or 5 mm in length.
Planaria are flatworms, related to the notorious tapeworm, but do not share their parasitic lifestyle. They are fascinating creatures often used in high school biology experiments because they will regenerate into two live individuals if cut in half. Nematodes are roundworms… they are one of the most successful and widespread group of organisms to populate this world (second only to bacteria… humans are far down *this* list)… nematodes are found from the far reaches of the polar ice caps to the hottest tropical forests to the depths of the ocean floor where no light has ever reached. Both types of worm are totally harmless to fish in themselves, and live on excessive nutrients in the tank.
However, although these worms themselves won’t hurt your fish, you may want to consider whether or not you are overfeeding or overcrowding your tank… these two factors are HUGE killers in beginner’s setups. To get rid of your worm problem, try not feeding your fish for 3 days (no, this is not cruel… healthy fish will do fine with an occasional short fast, and it will improve their water quality) and giving your tank a good cleaning with a 25% water change. If you keep small fish, some of them may actually enjoy chowing down on the little worms… guppies and platies, for example, love them. If you keep your tank clean, your filter well maintained, stay true to not overstocking your tank, and do regular water changes, your worm problem should be gone in no time.