I am not sure of the long term effects of keeping the fish in hyposalinity conditions. I've thought about it but dare not try More over at those salinity you cannot keep any inverts, like cleaner shrimps etc, quite boring tank right?
For 8 weeks only you complain of needing to check for pH & salinity. Then if you keep them at low salinity for long term then you'll still need to check for pH and salinity as well. Actually by just keeping the kH ard 10, the pH should remain pretty stable. As for the salinity just make sure the water level don't drop below a certain level will do the trick.
Copper is unstable in saltwater so you'll need a copper test kit to monitor the copper concentration, too high you'll kill the fish, too low you won't kill the parasites.