Currently I would recommend a large water change. 50% at least to cut down the nitrite levels. You will be doing a lot of water changes for a while to keep levels down until the new filter establishes. 50% every other day if not more. A better test kit will help you determine this. Stop feeding the fish for the most part, like once every 3 days or so to try to slow down the build up. For the future always transfer media when changing filters. Even if you need to shred up the old media and shove it in the new filter it will avoid water quality issues. Its usually recommend no more then 1/3 of the filter media be replaced at one time.