Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
You moved enough bacteria with the things from the established tank that the little blip of ammonia and nitrite is all the cycle there is going to be. The nitrifying bacteria and the plants are now dealing with the ammonia from the fish waste.
When you do a water change the ammonia that comes in with the tap water might register on a test for about 24 hours, but most of the time it should not show at all. The plants and bacteria should take care of it pretty fast.
Some ammonia tests are not compatible with some dechlorinators. If you can, get dechlor and test from the same company and see what they say about how the test will read for the first 24 hours after a water change.
After that the dechlor is mostly deactivated, and should not affect the tests.
Read all you can about the style of test you have, which reagents and about the dechlor.
If dechor and tests are from different companies, find out how they interact.
You can also calibrate your ammonia test by getting some distilled or RO water (no nitrogen in it at all) and adding small amounts of ammonia then seeing what the test reads, first of all testing the pure water, then testing it with some ammonia.
Common Plecos grow so large so fast they are not compatible with most aquariums. There are a couple of species sold as commons, and they can reach a foot to 2 feet long. I would return this fish. Look into the smallest of Loricariads such as Otocinclus, or the slightly larger Rubber Lip or Bulldog for your tank.