i do a 10% change every week, and a 25% every month along with changing filter media.
i just ran out of water test stuff a few days ago, all that i have is a pH tester and a NH3 which is at normal levels
Normal level = 0ppm, does it?
I would "+1" Fluttermoth's suggestion. Once the problem is sorted, I would also suggest changing more than 10% per week. I myself do about 30%, and I know others do more than me.
The other thing I wanted to say was about changing filter media. Many tank manufacturers recommend that you change filters regularly. This means that you spend more money on their products.
To bust a couple of myths for you
Biomedia (ceramic rings, noodles, etc.) never need changing.
Sponges don't need changing every 6 months, they only need to be renewed when they are literally falling apart. They just need to be rinsed in old tank water (NOT tapwater) to get rid of the gunk.
Carbon filters don't need changing every 3-4 weeks, they don't need to be in the filter in the first place. Their only benefit is to get rid of used medication from the water. Keep one in your cupboard for such an eventuality, and hope you never need to use it. If you do, leave it in there for a couple of weeks, then chuck it.
If you renew filter media, all you are doing is losing the beneficial bacteria you've built up, and this then needs to regrow, leaving you potentially with ammonia or nitrite spikes.