Because of the sheer size of those tanks going to type a lot of work to get corrected and very hard for some of us just going by what you're saying, need to go back to the beginning get a better understanding, first of all what is the condition of the rest of your fish would it be possible to have a few pictures of your Systems,
If I was in your shoes I would first do a full set of test and then correct them all as soon as possible and get every piece of equipment up and running properly including the calcium reactor, protein skimmer along with having a good clean out, Will give you a fresh start if you have a biological media it wouldn't hurt to give it a boost with a bio booster to help cope with the amount of fish that has gone in to quickly for your biological filter to handle, I myself would also do some very big water charges and keep doing them until things settle i'm not sure whether you use a synthetic salt or natural seawater so could be expensive but that is to be expected when you have large systems, your going to have to spend to get things right especially if you want 400 fish plus and depending on the size Will create lots of waste so your biological System will have to work hard.
Please note this is just the way I would try to correct things. And probably is many other Ways of doing it, if not better.[emoji846]
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