I'm asking a question on my mom's behalf...(this is for freshwater) She has a 12 gallon tank and it's been running for a year. She let it cycle a year ago and everything has been going smoothly. She changes her filter media every 2-4 weeks and does a 25% water change every two weeks. Recently she's been having an ammonia problem. The fish started getting fungus and she tested the water and ammonia was 1.0, nitrite 0 and nitrate 5 (this was right after a water change). For the next week she did frequent partial water changes and finally got the ammonia reading down to 0. She replaced the filter media and thought everything would be okay. She went on with her bi-monthly water changes. Her last water change was yesterday, and she tested today and her ammonia was 0.5, nitrite 0 and nitrate 5. What is going on? Why is the ammonia being so persistent? And what should she do about it? Her tank is not overstocked, it has 4 tiny fish in it and she feeds a small amount once a day. Thanks!!