I
thought you were going to ask that question.
I thought about it. I've known several people who did not successfully treat MI with copper and switched to hypo with success. So I don't think the pattern you had is unusual insofar as going from copper to hypo.
I hope that was a typo. Did you mean 1.008 or do you mean 1.018?
Microbe genetic variances don't quite happen like the concept of 'growing accustomed to' would imply. Amongst the MI organisms you had, one (or more) would have had to genetically preferred or could live at the low salinity when it was created. Natural selection would have favored that organism strain in the low salinity and it would continue to live and reproduce, making more microbes with the same genetic proclivities.
Like insects, microbes reproduce in very large numbers. Nature throws in a variance now and then (a random probability based upon the strength of that trait's genetic code) of a suppressed trait. In this way, nature allows the oddball offspring a chance to survive if the environment favors the trait that has shown up.
Like a black tree moth. Every 10,000 of them, one is white. On the black bark, the black moths are camouflaged from their bird predators. The white one gets eaten pretty fast! But a fungus breaks out on the trees one season and turns the bark white. Now the black ones stand out and the white ones are camouflaged. The white survives to go on to make more white ones to match the new environment. Nature no longer 'favors' the black ones, but that genetic trait is still standing by should the tree bark go back to black. :rollface:
Your copper treatment in a tank with sand will be challenging. Remember to monitor the copper content closely and keep it in the right range. There will be a loss of copper to the substrate/sand, and you should consider not using that sand again, after the treatment is over.
Obviously not all is well with you.

But I believe it will get better. I am fine.
Regards,