This morning I took the Emperor 400 HoB filter off of my 55 gallon tank and swapped all the biomedia in it into a new canister filter that I've had running for 5 weeks on that same tank. After that I did a water change of about 40 percent. This evening I noticed my featherfin catfish was hanging out upside down at the top of the tank with his mouth at the top of the water. This is not like him, he usually hangs out under some driftwood at the bottom. I immediately tested the water. My ammonia is 0 and nitrates are 5ppm but my nitrites look to be right between .25 and .5 ppm on the color chart. I am going to do a water change right now but wondered if this could be because of the filter change and how worried I should be. I thought having the canister hooked up for 5 weeks before completely swapping over was long enough, especially since I swapped the biomedia from the old filter to the new one. Any advice would be greatly appreciated.