Test some bottled water and compare it with what you are seeing from your tank water test. The bottled water will be zero ammonia, so if they look alike you have zero ammonia in your tank.
Sometimes test kits get out of date, dont read the same as they used to. Also, they arent all that accurate anyway. All sorts of things in the water can throw off a test result. Your test isnt even a test for ammonia, its a test for total ammonia nitrogen (TAN), which is free ammonia + ammonium and ammonium is not really anything to worry about.
On the face of it, 0.25ppm TAN is nothing to be worried about. At typical aquarium pH and temperature you need much higher levels of TAN before the free ammonia element becomes toxic to fish. Even at your pH if your water temperature is at say 25c you would need TAN at 1ppm before it became a concern.
If you are actually detecting TAN though and its not a false positive, it is a sign your cycle cant cope or there is a new ammonia source. Maybe something died and you didnt notice, maybe your fish grew and added more bioload and your filtration can no longer keep up, maybe you overfed, maybe your tapwater is now treated with chloramine (which is an ammonia source) whereas before it was chlorine treated, maybe your water company did some work on the infrastructure and put some additional treatment in the system afterwards. A bit of investigation might be called for, test your tapwater, if that shows anything maybe call the water company.
Maybe, if you havent been doing any filter maintenance its not operating as well as it used to. You do need to clean those things, you just need to clean them appropriately.
