[Uncle99]

Definitely agree with the ensure no copper thing, that's good advice.
The death of 5 out of the 6 chromis is very normal, I had 6 and two weeks later 4, now 1. Most reefers will tell you the same.....but the goods news is no new deaths.
Getting fish from sea to store to your home is a very stressful event. It is estimated that loss range in the 50%. It is during this process that the fish become week and diease sets in. That is why it is important to acclimatize from bag in terms of both matching salinity and temp in a QT where it is easy to control and medicate where necessary. In terms of salinity, the most common error with fish is not brining them up to your DT salinity slowly, that is .002 per day. Most LFS stores keep their fish at 1.017-1.019 to decrease the risk of diease. If you where to put this fish in your DT at 1.025, in 3 days to 1 week it would likely die.

IMO your really chasing numbers. Remember you said you changed all the water through a number of changes....this would not bring you to 100% of what it mixed to.

Since your only concern I see is ALK, and you need very little, just don't worry about it, but monitor the value after each water change...I do one 10% per week....then my tests and record in a worksheet.

To tell the the truth, I have no idea why your fish died, except as I said for the chromis, the most dominat killed the other 5, trust me.

But in order to move forward we must ensure the environment is correct, stable and unchanging, forget the numbers unless they are way way off. If you mix your salt to 1.025, that's a mix for corals, or 1.023, that's a mix for fish, then your number will stay correct if you do say a 10% weekly change. Only when you have a ton of LPS and SPS and clams should you need to begin to 2 part dose.

So again, after water change nothing has died, forget the numbers, and when ready maybe add 1 new fish (hardy and peaceful) acclimatize for both temp and salinity in a QT, give a week and make sure he's eating well....then off to the DT......and watch and wait.

I am hoping some of this helps, I chased ALK numbers like you, Mine mixes to 10, but 8 in the tank. I can dose to 10, but 24 hours later, 8 again.....then a guy on this Site called Mcgyvr......famous for saying don't chase the numbers gave me this advice and while I test, record, small variations I disregard provided the numbers are consistent. Consistency in reefing is most important.