Ahh, that's your problem, then.
The "webs" you saw were probably a bacterial bloom.
When you set up a fish tank with a filter and add livestock, the fish tank starts to "cycle." Your fish excretes ammonia through its gills and its waste. When a tank is cycling, bacteria which break down ammonia into nitrite, and nitrite into nitrate start to grow and multiply in the tank. Ammonia and nitrite are very harmful in small concentrations, while nitrate is not harmful in small concentrations and acts as a good nutrient for live plants.
You want to have enough of the bacteria to counteract the ammonia produced by your fish, in order to turn it all into nitrate. After a tank is set up for a bit, a bacterial bloom may occur.
In a cycled fish tank, you never want to replace the filter cartridge (that's where the bacteria live!). You only will have to rinse it off in old tank water that you've removed.
I recommend doing a large water change in your tank. Drain off 75% of the water, and replace it with clean, conditioned water of a similar temperature.
You can empty the water in two ways:
1. Siphon with a gravel vacuum or a bubbler tube. You'll want to vacuum up as much of the waste in the gravel as possible.
2. Stir up the nasties at the bottom of the tank and scoop the water out with a cup into a bucket.
From this point forward, until you're cycled (and even after if you wish), complete at least 2 50% changes per week. I recommend investing in API master test kit if you can afford it, in order to see how far along your tank is in cycling. A tank is cycled when ammonia is 0 ppm, nitrite is 0 ppm, and nitrate is 5-10 ppm.
After a tank is cycled, your goal is to keep nitrate in a comfortable range of 5-10 ppm with weekly 50% water changes.
Last edited by kfish; 02-08-2011 at 12:27 AM.