1.009 not all fish can handle it, i experimented with it to buy back fish i lost due to ich when i was doing hypsalinity. Anthias, wrasse, butterflies, flame angels, even a baby blue tang i had. They cant handle hyposalinity. What happened was in 2 days of being in hypo the fish would start to retain water, it would bloat to huge proportions and start to porcupine its scales if it had scales, then it would die. This is due to the fish drinking water like all marine fish do, but due to the lower salt in the water the kidneys were affected and caused the fish to retain water, this is what dropsy is bloating and pineconing of the scales. The only fish i saved during this was the blue tang by immediatly tossing him in normal salinity.. Yea i took a risk going from 1.009 to 1.021 but he deflated in 24 hrs. I even bought new fish to replaced the others that died thinking it was a fluke.. nope.. lost 3 flame angels, 2 butterfly fish, 6 anthias, and 2 wrasse from hyposalinity.
Its just like Naso tangs, my blond naso tang had what looked like black ich, ppl said to use prazipro.. NOPE i used prazi and he swelled 2x his normal size. I took prazi out and he went back to normal. If you do some extensive searches youll see naso tangs cant handle prazi for some reason, just like the fish for hyposalinity i noticed lots of people had the same issues.. same exact issue.. bloating and death. I even did hypo on a flame angel slowely.. took a week to drop down.. Once he got to 1.013 he started to pine cone and died. But other dwarfs angels seem to handle it just fine at least the ones i tried.
Both hypo and copper have risks.. But the risks outweigh having ich in the tank.
I will completetly disagree though on comments on maintaining hypo. You make it sound like its hard and just a tiny bit of evaporation will change the salinity drastically. at 1.009 it takes ALOT of water to change it even 1 point. Think about it. To drop from 1.009 to 1.007 for example. Youd have to change nearly 25% of the water with fresh water. Thats a 2 point drop with 25% of the volume. To go up to 1.010, 10% of the water volume would have to evaporate. In a 24 inch tall tank, thats almost 3 inches of water. So if you fill a rimmed tank to the lower part of the rim, the tank would have to drop 3 inches below that to even go up to 1.010. Just saying.