But bear in mind that you can't take the old filter out and replace it with another one. If you were to get the same sponge filter as me, it has a hollow core so you could put the sponge or whatever from the old filter into that core.
The current filter has a lot of beneficial bacteria living in the media. Without them, ammonia and then nitrite will build up and harm the fish so you need to somehow keep the old media. The usual advice to run both filters together for a few weeks won't work in this tank as that would create even more water flow.