I seemed to have gotten ahead of myself a few days ago in my post. I thought all was going well by adding my swirl filter, my water became very clear and free of media dust - I thought all was headed in the right direction. Yesterday I noticed about 5 fish (gold fish) floating in the top of the tank, so I removed them. Today after coming home from work, I noticed about 15 fish floating on the water. I quickly tested the water: pH mid to high 7s, Ammonia 0, Nitrite 0, Nitrate 10ppm. The only changes to my system is that I put jute around the tank (IBC System) to keep algae from growing in the sunlight and added a swirl filter. The water has a lot more movement now because of the outlet for the swirl filter falling back into the tank, but I dont think should have negative effects.... I did however noticed a small bubbled white foam (not a whole lot) around the edges of my tank and around my pvc coming out of the water from the pumps. This is the only thing that I can observe. Any thoughts or suggestions would be greatly appreciated.
Gold fish are are generally hardy. If the temperature didn't change or any hardness parameters, i would certainly look into toxins from the new equipment. I know you dont want to lose the N but slow water exchanges may in order.
It's hard to know without getting to see the fish. Examining them can sometimes help you figure things out.
Did the fish show any indication of what they died of (i.e. ich, dropsy, etc.)? That might give you a clue as to where to start looking. If they have white spots, it's probably ich. If they are bloated, or their scales stick out so they look like pinecones, or their eyes are puffed out, it's probably dropsy. If they have fuzz growing on them, it's probably a fungus. Those types of things would indicate stress from water quality changes. This can result from a rapid change in water quality for worse or for better. Did your water quality parameters (pH, ammonia, nitrite, or KH) change dramatically after you installed the filter, or was it just the clarity of the water that changed?
If there doesn't seem to be any indication of what they might have died of and there was no change in your water quality parameters, then I would think about the materials you used in the swirl filter as Tom mentioned.
It is also a remote possibility that the fish could have come from the fish store already carrying parasites and the stress of transport and introduction to your system got the ball rolling so to speak and it just took four weeks for them get sick enough to start showing symptoms and dying. This can be particularly true with ich infestations because of the lifecylcle of that particular nasty. Unfortunately, even fish from the very best fish stores can be carrying parasites and look completely healthy at the time of purchase and it just takes a stress event to lower their immune response enough to get the ball rolling.
Good luck with your detective work. I'd be curious to know what you come up with.