This is not in the best interest of either species unfortunately. You need to have 10 of the loaches for their well-being, which is OK here. You need 20 of the Celestial pearl danios for their well being. These numbers take into account the inherent requirements of the species.
The nitrate is too high for the fish being contemplated. Different species are affected differently by nitrate, and both the loach and the CPD are more sensitive. Nitrate in the tap water is one issue, and unless that water is diluted or other action taken, this is not going to lower in the aquarium. But it will increase, from the sound of things, and this is dealt with by more frequent water changes (keeping in mind the 20 ppm already there), substrate and filter cleaning, not overstocking and not overfeeding, and live aquatic plants, floating being ideal for this (and benefits for both fish species too).
@Essjay may know something of Knowle, I'm in Canada and never heard of them.