+1 on this sounding like an ammonia problem. Based on your description, it sounds as though this tank has had no livestock in it in 6+ months, has very little (none?) filtration, and the only input of organic material is whatever has been in your display tank water. Does this tank have a canister or HOB filter? Since your display tank is cycled, there won't be nearly enough ammonia in the water you're adding to your QT to keep a bacterial bed large enough to process the waste of three medium sized fish. In the 15 gallon tanks I use for the tank transfer method (which are filterless), 3 fish is enough to bring the ammonia levels to measurably dangerous levels in 48 hours. I have to keep that under control with rigorous testing and liberal use of Prime.
If you want to keep a QT tank ready to add fish at a moment's notice with little daily work required on your part, you need to keep it's cycle going while it's fishless. That means either tossing in a cocktail shrimp every so often, or dosing ammonia directly. Even then, you'll need to test for ammonia daily when you add fish, as the bacterial population will only grow to the size of it's food source, so there's a good chance adding fish will cause a mini-cycle no matter what you do.
Were you testing for ammonia? Did you test for ammonia when the fish died? Ammonia suppresses pH, so I'm not surprised your pH was low. In fact your low pH was likely why the fish lived as long as they did, and why one survived, as low pH keeps most of the free ammonia in it's non-toxic NH4 state. At normal reef pH, almost all free ammonia in the water would be in the super deadly NH3 state.
For a QT tank it's a good idea to have a quick and dirty API test for ammonia sitting right next to it, and the biggest bottle of prime you can buy on stand-by.
Also +1 on Prazipro. Best investment you can make I think.
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