Ammonia Levels Wont Drop Down

Which tester are you using? The API liquid tester is notorious for giving false readings, especially if you are looking at the chart under certain lights (mainly fluorescent, and compact fluorescent energy saving bulbs)

Can I ask what your pH is? The toxicity of ammonia varies with pH from the not very toxic at all to the quite toxic. In water, ammonia exists in both the ammonia and ammonium forms. How much is in each form depends on the pH. Ammonium is far less toxic than ammonia.

Are the fish showing any ammonia-poisoning symptoms such as flicking themselves against the decor or gasping for breath, at the surface in the case of the harlequins?

Have you done anything to the filter such as changing what is inside it?

When you've done the water changes, how low has the ammonia been straight afterwards?

The good news is that you can go up to 5ppm with your pH and still be safe. That's assuming the water temp is around 25 deg C which I forgot to ask  but I checked between 22 and 29 deg and it's still safe. It's at high pH that the ammonia is more toxic; with your acid pH virtually all the ammonia is in the less toxic form.

When you changed the filter did you keep the media inside it and put that in the new filter? If you didn't, you threw away all your bacteria and have been starting a new cycle since you changed the filter.

Can I check - are you measuring your nitrite as well? That is the next thing to go up if you are doing a new cycle, and that unfortunately is more toxic at low pH that at high pH.

Wait half an hour after a water change to allow the new water to mix in before testing.

The water wouldn't have had any filter bacteria in it as these bacteria live in the biofilm tightly bound to surfaces not free in the water. I would work on the assumption you are somewhere in the middle of a fish-in cycle until a continued zero nitrite proves you aren't.

Your pH and ammonia are still OK. I'll let you into a secret and tell you what I've been using. Look here. Leave the first box set at NH(NH3 + NH4); enter your ammonia reading in the second box; set salinity to zero; enter your pH then temp; then click calculate on the right hand side. Look at the lower box, NH3 concentration. As long as that box reads less than 0.02 you don't have a problem. The test kit measures both toxic ammonia and less toxic ammonium. That box in the calculator tells you how much of that total is actually the more toxic form.

Keep checking both ammonia and nitrite and only do a water change is the ammonia goes over 0.02 in that calculation, and/or you get a reading other than zero for nitrite. And watch the fish - if they start flicking or gasping, check those levels asap.