Goldfish stopped swimming and eating

Question
I have had my goldfish a little over 3 years and it has stopped swimming and keeps to one corner of the tank. She doesn't come up to the surface for food.  I got a new filter about 3 weeks ago and I noticed that the tank is unusually dirty.  What do i do?

Answer
It sounds to me like your filter is not functioning properly.  You really need to do a full tank cleaning with about a 50% water change at this point. Your fish may be suffering from ammonia poisoning/nitrate/nitrite poisoning or lack of oxygen. If you do not have an air stone in there, I would add one too for the extra oxygen.  Try this and get your water tested to see if the parameters are ok.  After it's done, see if there is a change in your fish, it could be that simple.  I would also check the filter closely or have someone else check it, something doesn't sound quite right.

Let me know what happens after this and we'll go from there.  Good luck!