0.25 reading of Ammonia is not good. Many things can cause "mini-cycles" in established tanks, for example;
a. Over-cleaning filter media. The only purpose of cleaning the filter is to keep it running smoothely- over-cleaning the filter though can deplete the beneficial nitrifying bacteria populations so much it causes the tank to mini-cycle. You should aim to only roughly clean the media, removing the heavy waste while leaving a bit of muck behind.
b. Cleaning filter media in undechlorinated water. Tap water contains chlorine and so undechlorinated water can kill beneficial nitrifying bacteria. You should only clean filter media in water taken from the tank during water changes which is then discarded afterwards.
c. Too many/too large water changes. While water changes are important to do to help keep the tank water physically clean, lower disease levels and help prevent build up of undesolveable minerals etc, you can do too many water changes or too larger ones. The beneficial nitrifying bacteria needs a constant supply of ammonia to survive & thrive and so when you do a water change you temporarily lower the ammonia levels- so doing water changes too large for the tank (generally speaking larger than 60%) or water changing too much (like every day or every other day, which you should only do when the tank has water quality problems) can upset the ecology of the tank too much.
d. Big cleaning session: If you have not cleaned your substrate & decorated area's for a very long time and they have collected a lot of waste in them, removing all the waste at once can stir up hidden ammonia. If a tank has not been properly maintained for a long time and has become quite filthy, the best plan of action is to clean the tank in sections or stages to prevent this.
e. Adding too many new fish at once.
f. Inadequate biological filtration: The majority of pet shop fish are sold at juvenile size, and sometimes while the tanks filter can easily handle the bioload of the juvenile fish, as the fish grow and produce more waste the filtration struggles to keep up with the increasing bioload of the fish.
g. Rotting debris/food. Depending on the decor of your tank, certain zones may be prone to catching fish food in them which then spoils before it is found and eaten by the fish (fish won't eat spoiled food, and foods like fish flakes & pellets can spoil quite quickly in aquarium water). Rotting fish food, dead fish, dead plants etc can all add ammonia to the bioload of the tank.
Etc.
