Major problem in my saltwater tank. HELP?!?

So there is still tiny white moving specs invading my tank gl and water. I've had fish5 fish die in 2 weeks. First there is tiny white specs on fins n body then in like the matter of hours it's piled on there heads and bodies and looks like dust. All the levels are good in my tank but there is constant red slime algae everywhere. The nitrate is low. Now my Cardinal fish's tail and fin is swollen loosing color and it looks like some of the scales are broken open on top. I've lowered salinity I've been doing water changes and I've been treating the tank for like 3 weeks with no results. HELP!