Strange indeed, I'm afraid at this point unless someone else comes in and has a great idea a completely start over might be necessary if you don't see nitrites or nitrates in a week or two. As afremont said give us the exact name of dechlorinator you are using and check for any expiration dates. Only other thing I can think of is to look up your water districts website and post it here so we can look over the quality report to see if anything is disturbing the cycle that way.
Besides that if I was in your shoes I would look for a complete start over, meaning find a temporary hold for your fish while you completely tank everything out of the tank (gravel, decor, heater, filter, everything) and clean it really good with tap water then let it dry and do the same with the tank. Then start completely fresh, add water, heater and filter and dechlornator. Then add fish back in but not any decor or gravel until the cycle starts (you see nitrites), just to eliminate those two from the possibilities of the problem. Don't add anything else besides what was listed, such as no bacteria supplement or any more fish. And hopefully you can start seeing some results.