Help! Tiny Wrigglers in tank

[Alan]

One Thursday night my Giraffe catfish didn't eat his supper, which is unheard of! I didn't feed him on Friday night, and he only had a very small amount on Saturday night. He only had a very small amount on Sunday night as well. Otherwise his behaviour is normal. On Sunday afternoon I noticed a couple of dozen tiny wrigglers in the tank, but couldn't get any decent pics of them:

The next three are the original, and two others I altered the contrast and levels on to try to improve them:




After finally managing to catch a couple, they are practically colourless (unlike in the pics), range from 4-10mm long, and are certainly less than one tenth of a millimetre wide.

Anyone any ideas what they are, and how to get rid of them if they are nasties?

I haven't added any new fish to the tank for nearly a year, and I can't see any in my other tanks. Levels of ammonia and nitrite are 0. Fish in the tank are Giraffe cat, black ghost knife, Syno Decorus, S. Pleurops, S. Hybrid, P. Costatus. Food is dry (flakes/pellets/koi sticks) and frozen (tiger prawns, mussels [for human consumption!] and usual frozen blocks of bloodworm, squid, daphnia etc)

[Mike_Noren]

There's a possibility they're catenulid flatworms or freshwater polychaete worms, but in all probability that's small aquatic earthworms, relatives of Tubifex basically. In either case they're harmless.

You got them when you bought plants for your tank; species like them are pretty much guaranteed to exist in any aquarium with plants, most of the time people just don't notice because these animals are small, transparent, and spend their days down in the sand, feeding on organic debris (fishfood, fish feces, dead plant matter).

As to why yours are now on the glass... One can usually find one or a few on the glass at any time, but you seem to have quite a lot. Either you have algae growing on the glass, which the worms eat, or oxygen levels in the sand has nosedived. Anoxia in the sand will send anything living there - snails and worms mostly - up the sides of the tank to escape asphyxiation.

EDIT: your catfish's unusual behaviour could also have something to do with unusually low oxygen levels in the aquarium. Maybe time for a water change and/or improved filtration?

[Alan]

Your reply has reminded me; I have fed live (dendrobena) garden worms to my fish, bought from an angling shop. The wrigglers are not on the glass, but are swimming freely in the water. I do a water change every week, either about 120l or 250l at a time.

My substrate is mainly sand with some ~3-4mm gravel.