In the OPs second post they say ph is 7, ammonia/nitrate/nitrite all 0.
i dont think its the air pockets. i have never once seen anybody say that the air pockets from their substrate killed their fish.
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Not really an air pocket but a gas pocket that is highly poisonous.
And you really never once have seen this happen to anyone???
Its happened to myself. I can point you to a couple other forums where others have had it happen. I can honestly tell you it can and does happen.
While it is possible to buy locally bred and raised GBRs, it is still far more common to find fish-farm raised GBRs that not only recieve a bunch of antibiotics, but steroids as well. Captive bred does not neccessarily mean healthier. While with discus it is absolutely the case that the majority of domestic discus are much more hardy...it varies greatly from breeder to breeder. It is the same with GBRs...captive bred does not always equal good quality stock.
I find your water parameters curious....you have no nitrates. Even in my planted tanks I ALWAYS have nitrates. Either your tank is not cycled or your test kit is not working properly.
from what ive read and experienxed it unlikely.
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I also find it odd that there are 0 readings all across. I keep the belief that you will ALWAYS have at least nitrates in a cycled tank. Is the OP adding water close to the same temp. as the actual tank? if the temp of the new water is too different, it could send the fish into shock(thats what it sounds like to me)