I am by no means an expert on this my friend, but here is my take on the circumstance.
I personally believe your tank is a bit stocked, perhaps in relation to a smaller footprint housing fish with such a high bioload. Without knowing more about what houses bio in your tank I am going to guess you were on the verge of enough -> not enough bacteria to prevent small ammonia spikes already. Personally I think what started this business is you have one particular ray more sensitive to ammonia spikes, even small spikes such as due to heavier feeding, than others. All it takes is a couple of these spikes for a sensitive ray to become stressed, go off of food, and from there it is a slippery slope and a bit of an uphill battle in the sense that you have to do more to correct it than what little issue caused it.
I know water does not have a lot of properties that hold this beneficial bacteria, but there is definitely some of that bacteria in there. Picture this, you have a small bio problem, and what you then do is take out a chunk more of the bio by doing the 80% change. Before you adequately treat the tank (that is the prime/conditioner was added after the fact, plus it takes some time to disperse around the tank), some of the chlorine in your water likely killed a bit more of the beneficial bacteria. This normally is completely negligible, but if you indeed have a small bio issue this could definitely compound into a mini cycle.
So a bit of math here: previously stressed out ray + large change in water environment (one way or the other, good or bad, it was a change) adding to stress + now you have ammonia readings. Not going to finish the equation yet because we don't have all the variables. My point is you have a stressed ray, and other fish that are not as stressed. At this point your job is to do as much as you can to stop the stress of the ray, and I feel like big water changes aren't the ticket, that small and more frequent ones are. Pure speculation here, maybe you don't have a bio issue to begin with, but ask yourself, if you replaced 100% of the water, which is close to what you did, would you expect your bio filtration to IMMEDIATELY treat all the water? Gotta remember the bio loads on these fish are massive, and I bet over the years of you keeping them the feedings have not gotten smaller. One way or another you have to pinpoint the source of the ammonia, and you can't do that accurately right after a large change. Mini cycles are not fatal to rays, long periods of sulking and refusing to eat can be.
This is just my speculation here man, take it with a grain of salt, I am not a doctor, and I am by no means considered an expert at the nitrogen cycle or fish keeping. You could also tell me you have way more than enough bio media and that I should go fly a kite, which could be true, but somehow if your test kit is accurate you are getting an ammonia reading.
Clean those fx6's if you haven't, make sure there isn't rotting food somewhere in your mechanical or in your piping. I personally would follow my above suggestions, but what works for one person doesn't work for others. Has your pH been steady as well? What is your normal reading? A large water change can result in a pH swing as well if you don't have a regular drip or a buffer. Some rays are just more sensitive than others man, and as we have learned in others threads, sometimes it is best not to overdo anything, just go into stress reduction mode and leave it alone for a while. Keep an eye on parameters and keep us posted. Good luck.