What is this worm? Not planaria!

Question
I have a fishtank of about 6 US Gallon capacity. I do not know PH, nitrite and ammonia, but it is an established aquarium, with 20% weekly water changes, so I guess Ammonia should be close to 0. I add enough marine salt so that the water tastes just slighty salty. Temp is kept at 81F and light is 12 hours of normal fluorescent bulb.

The aquarium is home to 3 generations of mixed molly fry, the largest at about 13mm long, the smallest at 6mm, to a totall of 16 fish. There are an undeterminate number of ramshorn snails of all sizes, and some plants.

I have a corner filter and an undergravel filter, and I do tend to overfeed, since I want to make sure the smallest fry have a full stomach. The snails help with most of the leftovers.

Lately I noticed some very thin, off-white worms that swim around when i disturb the gravel. The fish love to eat them. The worms range from under a milimeter long to about 6mm, and are thinner than hair. They swim in beautifull almost perfect sine waves.

I tought they were planaria, and so did people on most forums. I could not get a full worm, they are fragile and hard to catch, but I link to an illustration I made of an almost complete worm I could get under the microscope. At 40x it looks segmented and round, not flat. One end was missing, not sure if it was the head or tail. The remaining end had no eye spots. Every segment has a pair of 'hairs'. The hairs have very little independent movement, not like cilia or legs, they move with the body. The worms twicth and curl into a ball when disturbed, and after a few minutes under the microscope they start turning red and die.

Do you have any idea what this might be?

The illustration is at

http://www.overcaffeinated.net/users/nepsis/GUSANO.gif


Answer
Hi commodore64;

That's a pretty strange lookin' critter! I don't know what it is if it isn't planaria. As long as the fish seem to like eating it and they aren't attaching to them, I wouldn't worry about it.

You might even try posting about it over on the freshwater fish forum at about.com. You can even post the picture there. Membership is free and the folks there are great. Here is the link;

http://freshaquarium.about.com/od/questionsanswers/a/naavigateforum.htm

Followups welcome

At Your Service;
Chris Robbins