Karawr, I have a single XP3 filtering a 120 gallon, 6 foot long, tank by itself. It will not prevent any waste solids from building up on the surface of my substrate but it has more than enough biofiltration for my tank. I run that tank rather heavily stocked and never have any problem losing fish in it from chemical poisoning, although I sometimes see a fish die of old age at several years in my tank. I have no idea what you are seeing that leads you to believe that the ammonia processing suddenly stops at 0.25 ppm of ammonia unless you are reading the results wrong. Even if the filter were not performing well, a test a few hours after a 0.25 ppm result should read a solid zero. I only trust my API kit result when I read a test tube with the tube touching the white area of the card and a sun light source from a window over my shoulder. Fluorescent lighting can distort colors and direct lighting can make a color hard to judge. If you walk out into the sunshine and try to read a color, it will be so washed out you won't be able to judge it at all. That is why I stay indoors and let the light come through a window at my back. I want good light, not extreme light. I read the lightest color that I can see with that tube against the card and compare it to the card's colors. More often than not the result is quite obvious to me when I do it that way.
If you want to test tap water, be certain to dechlorinate it first. Chloramine is a combination of chlorine and ammonia, so breaking that bond can result in seeing some ammonia. In my own tap water that comes to about 1 ppm of ammonia in dechlorinated tap water. Since I treat my tap water with Prime, I do not worry about the traces of ammonia that are left behind from my tap water. The ammonia portion is processed by my filter before it can revert to the more dangerous form of ammonia.
