Have you reached out to the R2R community to have some experts review your QT setup and procedures?
I personally love Live Aquaria and have had amazing turnout with my QT. I lost 1 fish in QT out of 16.
Happy to help if I can, although an expert I am not!
I'm happy to share my QT procedure. Its the same procedure that I use for fish I have bought from Mini Reef, and never lost a fish yet from there...at least during QT. I had 2 fish carpet surf, and that's on me for not having a screen.
When I get the fish (we'll use LA since this is the discussion) from LA, I float inside my QT for 30-45 minutes, maybe long depending on my work call schedule. After the specimens have been temperature acclimated, I pour the contents into a 5g bucket, and immediately dose with Prime to combat the sure to follow ammonia spike. I test the salinity, and from LA it's usually 1.016, and I run my QT at 1.025. I start drip acclimating, and every 15 minutes, I take 2c water out, and put in 1c of tank water. After 45 minutes, I test the salinity, and this keeps going until I get to 1.024 or 1.025.
Once I reach salinity, I use a colander and scoop up the fish, and place them in the QT tank, with lights out. Lights out for 24 hours. At this time, I also pour in a bottle of BioSpira to help. In addition, I have bioballs that are seeded from the display, so I should have plenty of bacteria. After 24 hours, I turn the lights on, and give a little food. If they eat, I'll make my first dose of chelated copper (Copper Power or CopperSafe). The dose is 25% of therapeutic. Every day, I'll add in 25% until I hit therapeutic and then my clock starts. No other medication is in the tank at this time, and honestly the fish have no issues with this. I observe no change in behavior while the chelated copper dosing occurs. They fish are happy, swimming, and eating.
Per Live Aquaria's instructions, I change 2.5 gallons of water every other day, and dose the proper amount of chelated copper to the new water. I test my copper readings with the Hanna High Range Copper checker twice a week to make sure, everything is good.
Now, with Live Aquaria Fish, I'll always have 2 casualties within the first 3 days. it never fails. Things are good for the next 14-17 days, and about day 20-21, with no changes to the procedure, if I have a group of fish, they will start falling by the wayside. By the end of the 30 day treatment, no live aquaria fish is alive for a 14 day medicine free observation period.
Fish I have that have gone through this, 4 clowns, 1 midas blenny, 1 bi-color blenny, 1 yellow tang, 1 tomini tang, 1 hippo tang (the bi-color and 2 clowns are in the biocube). No issues whatsoever. Something about this procedure doesn't work with Live Aquaria fish.
I know the fish I get from Mini Reef, their holding tanks are treated with copper, they tell you about it.