IMO, your pH is way off for GBR. I cant believe so much difficulties with GBR.
I used tobring in 100-200 from SE Asia and very rarely had an issue as long as they arrive alive. Would have GBR pairing off in holding tank like there is no tomorrow.
I am sure some would say fish would adapt but adaptability has wide range of factors which can contribute to massive losses and/or complications. Another word different fish w/i same species and even same batch can react differently to extreme water condition.
Provide ideal condition for fish being kept in q/t (check lfs /breeder's water first).
Once well acclimated to ideal water and if keeping ideal condition is cost ineffective or difficult to prepare, then you shoud slowly change the water condition over long time (I would rather provide ideal conditions).
Is your tap high in pH/GH? And if you are using common pH test kit, pH of your water could be higher than 7.6 unless you are using high range kit.
When fish are in quarantine in extreme conditions, unnecessary medication itself/themselves are stress factors which can contribute to further stress and may lead to death.
As you said, colorful one die of no reason.
There is always reasons, we just dont know for sure. Thus at least we can rule out water condition as possible culpit when given ideal condition.
GBR was selctively bred and thus stronger and colorful than common ram (at least shipped lot better than common blue/gold ram).
GBR would breed like crazy when kept in ideal conditions and with minimal complications and/or diseases.
Cut out all these med. I believe this med is more of disinfectant (oxidant) similar to potassium permanganate.
Personally I would not use such chemicals for prophylactic purposes. And I also dont believe one cures all. It tells you right on the warning that this is not for scaleless fish. This should tell you it can very potent and even toxic to some fish.
I may use it as sterilizing agent for nets, dried tanks, etc. Even then I may not even used it when I can sun dried my equipments/apparatus.
BTW, only thing i will use for prophylactic purpose is ideal water condition for specific fish.
If you want to try another batch, check supplier's water for pH, NH3, NO2. Sometimes Ill bring my own test kit to verify the condition being kept. And go from there.
Pardon me if I was too harsh but i am very frustrated with so many GBR losses and complications Ive been reading on few forums.