I see you just joined this month, so welcome to Tropical Fish Keeping forum.
Now, to business; I am sorry to say, you have some major issues. The fish you have are inappropriate for a 12 gallon tank, and the high nitrates are one result. We have fish profiles here, second tab from the left in the blue bar at the top of the screen, or you can click on the fish name when it appears shaded in posts [which it will if the name is the same as that in the profile]. Please read the profiles of these fish, I won't repeat all that here.
Potentially large fish cannot be kept in small tanks except as a very temporary measure. Fish grow all their lives, and often when young may have growth spurts. If the environment is not sufficient, they can become stunted, or deformed internally, meaning they will have endless health issues down the road and assuredly shorter lives. Part of this is the stress caused by the small space and resulting water conditions; stress weakens the immune system in fish just as it does in humans. Another aspect is keeping shoaling fish like rainbows and loaches singly; without a group of their species, this is very highly stressful, so it adds more to the problem. Loaches in particular have a high social structure within a group, and this has to be met.
With a pH of 6 your ammonia is actually ammonium, relatively harmless. Bacteria use ammonium/ammonia regardless, and if you have live plants they will use ammonium as their preferred source of nitrogen. However, the problem is that if there are no live plants, nitrosomonas bacteria use the ammonium and produce nitrite, and nitrite is also highly toxic, so this will harm the fish if the nitrospira bacteria are not in sufficient numbers to handle it.
When you say "stress coating the water" are you meaning the use of StressZyme? This will increase ammonia further, as it affects the bacteria and biological system in the tank. And I agree that Cycle should not be used regularly, if at all; it is chemical, not bacteria like bacteria supplements such as SafeStart (Tetra) and Stability (Seachem), but even those should only be used at the start of a new tank, not further; a healthy aquarium must have an established biological system, and developing it naturally is far better than disrupting it with additives. The only substance that should go into a cycled tank is a good water conditioner and plant fertilizer.
The biology of this tank is out of balance, and stressed fish produce substances (pheromones) that further induce stress. My advice would be to remove these fish from this aquarium either by setting up larger appropriate-sized tanks and increasing their numbers accordingly, or returning them to the store or another hobbyist. I am sorry, but the present state is only going to worsen as these fish continue to "attempt" to develop. In the meantime, daily 50% water changes with a good conditioner will alleviate some of the trouble for the fish.