One of two fish in copper quarantine died, next step?

[Halo_003]

So at the beginning of the week I started a quarantine for two Bangaii cardinals, in a 5 gallon tank predosed to 1.3PPM with Copper Power.
I've been slowly ramping up and reach 2.5PPM today, but today I found one of them dead. Given that the survivor is now alone in the QT at 2.5PPM copper, would it be a reasonable idea to ramp back down to 1-1.5PPM, add a replacement, and then ramp back up back to 2.5PPM? I know this would reset the clock on the quarantine, I'd just prefer to get a pair into the display tank together.

Or would it be smarter to carry on with the one and then run the other separately?

I am sure it wasn't an ammonia problem, the biofilter is 250ml of Seachem Matrix that sat in the DT sump for about 6 months in a drawstring bag, and I have an ammonia alert badge in the tank as well, which currently shows "safe". It isn't an oxygenation issue either, I have a lot of surface disturbance. My suspicion is it was something internal, because I looked at it pretty closely and saw no obvious harm to it externally.

Copper PPM is based on the math, I have a Salifert test kit which I don't think gives accurate readings (it says between 0.5 and 1.0 PPM). I have a Hanna checker on the way from BRS but it isn't supposed to arrive until the 20th.

 

[PSU4ME]

If you don’t have a Hannah checker yet then I might opt to remove existing copper and get a second fish then give it another go. You want to go slow and shoot for right around 2.0 ppm, if you were guessing and saying 2.5, you could have been off and killed the fish due to the concentration being too high

 

[Halo_003]

If you don’t have a Hannah checker yet then I might opt to remove existing copper and get a second fish then give it another go. You want to go slow and shoot for right around 2.0 ppm, if you were guessing and saying 2.5, you could have been off and killed the fish due to the concentration being too high

Thanks, that's a good point on just waiting for the checker. I don't think the concentration went too high though because I only dosed 7.2ml in the 5 gallons of water, which puts it right at 2.44PPM according to the Copper Power dosing ratios.. I think another possibility is a 3.5 day ramp period was too little.