15 gallons of 12.5% chlorinating liquid (assuming that is what you used) in 200,000 gallons would add 9.4 ppm Free Chlorine (FC) to whatever level you started with. That is more than enough to achieve breakpoint with monochloramine. In fact, the 10x rule isn't even correct, but I won't get into that here. The problem is that the Combined Chlorine you have in the pool is probably chlorine combined with other organics (not ammonia/urea) and those may be hard to "break" and can result in persistent combined chlorine which is what you are seeing. This is rather common in indoor pools, possibly due to the lack of sunlight (since UV may help break down some combined chlorine) and possibly due to a lack of Cyanuric Acid (CYA) in the water (more on that later).
As Dave pointed out, the use of a non-chlorine shock, potassium monopersulfate (MPS), is normally done to prevent the formation of combined chlorine and can, to a lesser extent, be used to try and remove combined chlorine though it isn't particularly good at that "after the fact". There are also "designer" MPS-like products that can very powerfully remove virtually all organics from the pool (see
Truox, for example), but both MPS and these designer products are expensive.
Now, back to CYA. CYA is not normally used in indoor pools because it is thought, mistakenly, that its only purpose is to protect chlorine from breakdown from UV rays in sunlight. Though this is one role for CYA, it is not the only one. CYA combines with chlorine rather strongly so that at normal ranges such as 3.5 ppm FC with 30 ppm CYA, about 97% of the chlorine (FC) is in the form of chlorinated cyanurates -- that is, chlorine attached to CYA. Only 1.5% is hypochlorite ion and only 1.5% is hypochlorous acid (at pH near 7.5) with only the latter being the effective disinfectant and oxidizer. ALL reactions with chlorine are slowed down by orders of magnitude when CYA is present. Or put another way, ALL reactions with chlorine are sped up significantly when CYA is not used. This includes destruction of swimsuits (especially reduction of elasticity of rubber and fading of non-fade-resistant fabric), flaking of skin, frizzing of hair, corrosion of metal, and generation of disinfection by-products including nitrogen trichloride, chloroform, and others.
I don't have any proof of this in real pools (yet), but I believe that using a small amount of CYA in indoor pools, say 10-20 ppm, will significantly reduce the aforementioned problems. My wife experiences the degradation of swimsuits, skin and hair problems when using the community center indoor pool that are noticeable during even one winter season of use with 1-2 ppm FC and no CYA while no such problems occur in our outdoor pool with 3.5 ppm FC and 30 ppm CYA over 4 summers of use. What I don't know is what will happen (in an indoor pool with CYA) with Combined Chlorine over time. My hunch is that the production of CCs will be slowed down, especially those persistent CCs that I believe take a while to form in the first place which is why they are so resistant to "break" through simple addition of chlorine (i.e. the activation energy of formation is relatively high so it doesn't reverse or go forward very quickly). If the effective disinfecting chlorine (hypochlorous acid) concentration is lowered, then the formation of CCs may get slower as well. It doesn't eliminate the problem, but may make it manageable through normal dilution (water replacement) which should be done in any commercial pool over time anyway due to the buildup of many substances, including salt.
So I don't have good answers for you on solving this problem, but may have a way of reducing it in the future if you are willing to try it out. It may be that slowing the formation of CC will have the existing CC drop over time, especially with some dilution and air exchange. Of course, once one adds CYA to water, one cannot remove it except through dilution or through use of one of those designer MPS products that are exceptionally powerful (fast) oxidizers (essentially MPS with a cobalt catalyst), so if I'm wrong it's not easy to undo the "experiment". That is part of the reason no one has tried it (yet), but I'm still hopeful. By the way, at the other extreme, there are some indoor commercial (typically hotel) pools that use Trichlor feeders and have too much CYA. This produces other problems, mostly too slow a breakpoint of ammonia/urea which leads to very high monochloramine levels that can be irritating. So too much CYA is bad and no CYA is bad but a moderate amount is just right -- sounds like Goldilocks and porridge.
Richard