It's a good idea to get a test kit if you don't have one so you can be sure if it's ammonia or not. That's really the only way to know if 25% water changes every other day will be sufficient. Also, since it's divided and there are two fish the ammonia is still going to build up faster anyways.
I don't think that is necessarily "weird". Some fish just react differently to things than others. There are odd fish who can survive a long time in water that is never changed. I'd monitor him since you just did a water change to see if he gets better and watch for other symptoms.