All seems well enough, now for the ammonia. The main thing that I can think of is a possible dead fish rotting somewhere in the tank. If you have lost a fish and can't find it, that can cause the ammonia to go up.
The second thing I can think of is you just might be doing too good a job of cleaning the tank. For a tank to cycle properly, there needs to be a point where all the nitrate/nitrites and ammonia get really high and that leads to a larger colony of beneficial bacteria growing. In your case, that size of tank with what few fish you have would take a fairly long time for the waste products to build up. Maybe you are at the point where ammonia has built up? I'm not totally certain about the Nitrogen cycle so this may be false information. You might want to look it up to get a better understanding.
I can't say anything for sure, you should probably wait for someone with more experience in this area to comment.