Fish tank slime

Question
I've a 24 litre cold water fish tank (Rena) that has been setup for about 5 weeks with 3 goldfish (Orandas I think) and one pleck. Within the last few days the normally clear tank has developed a almost colourless slime on the tank walls and filter. This morning I found the pleck dead and the goldfish seem very sluggish and didn't eat, both of which is very unusual. I have some water quality testing strips which give me info on pH, nitrates, nitrites and water hardness. pH was about 6.8, nitrate level was below the recomended level of 50mg per liter (I think that was the units), the tester strip said it was about 25. No nitrites and water hardness about 10. All of these values were within what the book that came with the tank recomends. I assume that the death and the fish behaviour are related to the slime as they occured at the same time. Do you have any ideas of why I get the slime and if its related to the problems I'm observing  

Answer
Hi Tim;

The slimy stuff is probably from the goldfish. They are very messy fish that have an exceptionally thick slime coating on their bodies. When they are overcrowded as yours are, it will often show up on the tank surfaces. Get a much larger tank with good filtration and things will get better. It is normal for established tanks to have a bit of "slimy feel" to them anyway. It's from the beneficial bacteria clinging to surfaces.

Goldfish also get very large so they require lots of room. So do plecostomus. A 24 litre tank is really not even big enough for just one pleco or goldfish to live in for very long. They each need about twice that size. Plecos get to be over a foot long and goldfish about 6 or 8 inches each. Here is more about goldfish and their needs;

http://www.firsttankguide.net/goldfish.php

A page about plecostamus;

http://www.fishprofiles.com/files/profiles/411.htm

At Your Service;
Chris Robbins