Regardless of water parameters I would not use water from one tank in another as 'water change' ..Been there done that and it did not work out.. Fresh water is better IMO
As for 'dusting' on fish look into marine ich and pray it is not...Ich in marine systems is a very tough issue..Completely different and way more trouble then freshwater .
I would try new water and getting NO3
to 20 or less before adding new fish..
How are you acclimating the new fish ?
Many fish stores keep their fish in 1.020[boarderline hyposalinity] to curb ich outbreaks specifically..So drip acclimating the new fish over a couple hours to adjust to your 1.025 might help also ?