In a tank of that size, ammonia can build up very quickly. It wouldn't take a decaying plant or much fish waste to cause it. I don't know if you could even maintain a cycle on it.
What's your WC schedule? You have do to 100% changes fairly frequently to keep the ammonia down. Others will chime in on this, I'm a big tank girl.
Actually, a .25 ammonia reading isn't bad, but it shouldn't get above that or you risk hurting your fish. You have to do WCs and gravel vacs as needed to keep that number at or below your current reading. Fortunately on such a small tank it's not such a big deal to change water.
My schedule *had* been 50% once or twice a month, but no more! I hear twice a week is the way to go, I think I'm going to wash the gravel thoroughly and then go to that schedule.
When I had a 2.5 gallon tank I did two 75% water changes per week with a gravel vac (helps get all the junk if you keep stabbing it into the gravel). and a 100% change- where I took everything out and rinsed it out- once a month.
Well, that's your issue. Do at least two changes a week, of at least 50% each (but will probably have to do more on one of them).
Do not take your gravel out and wash it, just use a gravel vacuum. Taking it out and washing it will only cause you to loose the little beneficial bacteria you have in the small tank.
Thanks for the info. I *have* started changing the water twice a week (have been doing it for a couple of weeks now) but the ammonia has still not gone down.. do I need to be patient and just persevere a while longer, or is there something I need to do first to get it to zero?
Without a cycle it will never be zero as fish always produce waste, and it doesn't take much of anything in that volume to get a reading.
Fast growing stem plants can really help, and even more so floating plants. The Java Fern is nice and you should keep it, but they are a very slow grower so their effect on water quality is correspondingly low.