This is my 16 gallon bowfront. I'm running a Fluval 206 canister filter on it. It has been established for several years now, and is usually crystal clear. About two weeks ago it started gradually getting cloudy. Last weekend, I did a partial water change, and rinsed out the filter media in the siphoned tank water. There was no noticeable improvement, so two days later I replaced two of the four course foam blocks, replaced the polishing pad, and added a bag of carbon. I also added a small HOB filter I had just for fun. Now, a week later, it appears worse if anything. I tested the water this morning, and the only thing I see is the nitrates are high. (Yes, I know stick tests are notoriously unreliable, and I do have an API test kit. I did this just for a quick look. I don't need a lecture on test methods.) Any suggestions for my next steps? I don't think it's overstocked. I have a pleco, two cories, two neon tetras, and four tiger barbs. No fish have died recently.