I've had a freshwater 23 gallon aquarium set up for 3 months. I have 7 black skirt tetras and 10 assorted platies. I use an a.p.i. filstar xp 1 and an aquaclear 30 for filtration. I have 3 small air stones and artificial plants. Every Sunday I change 20-30% of the water by way of a siphon. Replacement water is treated with a.p.i. water conditioner. I test the water with an a.p.i. master test kit. My nitrates are 0, nitrite 0, p.h. is around 7.5. But the ammonia level is always very high. My fish are active and do not appear stressed or unhealthy. I'm confused about this problem. This past week I purchased a bottle of ammo-lock. Of course ammonia still registers high, but supposedly it's converted. I would be most pleased to receive any advice.