threadlike parasites in water?

Question
Tank has been set up for 5 years.  It is a 50 gallon tank with a magnum filter.  Changed water 2 times a year with 50% new water added.  We had a plecostomus, 2 gouramis, 3 serpae tetras and 1 angel fish. There are white squiggly worm like threads floating around in my 50 gallon tank.  It has slowly killed off all but one of my fish.    Slowly 1 by 1 started to die. They had gotten bloated and their scales started to stand out, then they died.  The plec had red blood blisters on the fins and scales.  We had only one serpae tetra left and this one also got the same symptoms and then died.  The tank has been empty for over 6 months.  We are still seeing the same squiggly worm like things floating around in the tank.  We thought that after the last tetra died the worms would be gone.  However they seem to be surviving on the algae in the tank.  Just wondering what they are and is there any way to get rid of them?  

Answer
Hi Marko;

The worms are simply a type of planarian or nematode and are harmless to your fish. They don't feed on living tissue or algae, they are thriving on excess waste. The only way to get rid of them is to starve them out. There is no chemical treatment that kills them. They even survive bleach treatments! They are only a symptom of the real problem anyway, which is dirty gravel from lack of aquarium maintenance. After 6 months and you still see critters, it must have been really dirty in that gravel.

Weekly maintenance with a water change of 25% and gravel vacuuming every week to two weeks is very important for all tanks. Even if the tank "looks good" and the water is clear, the invisible waste toxin known as nitrate (NO3) is there and slowly building up. The water becomes becomes old and depleted of important minerals, trace compounds and electrolytes as time passes so the fish can't fight off any disease that comes their way. Disease pathogens live everywhere waiting to attack and your poor weak fish just didn't have a chance I'm afraid. The water changes we make in our tanks are what keeps the tank and fish fortified with what they need to keep their immune systems strong. The high nitrates (dissolved organic waste) that built up in there over several months caused organ failure in the fish leading to fluid build-up known as "dropsy" and infections.

If it were my tank I would drain it, rinse the gravel and decorations to get rid of the worms and the excess crud and replace the filter media. Basically, start over. You now know how to properly maintain the system and can keep your next bunch of fish happy and healthy. It will have to go through a break-in period. Here is a link to my article about it to help you get started and through the initial 6 weeks or so safely;

http://www.xanga.com/Expert_Fish_Help

Better luck next time...

At Your Service;
Chris Robbins