always getting sick

[mark]

if your tank has had ick, there's a chance every fish you add it will catch it no matter the quarantine, as the ick is in the tank.

Keep the fish healthy as you can, keep the stress down, keep the water quality up will help reduce outbreaks but won't eliminate ick. For that you'll need to remove ick hosts from your tank (fish) then let the tank go fish fallow for something like 6 weeks to break the ick life-cycle. Then to ensure you don't get ick re-introduced you need a rigid quarantine of everything that goes in (fish, corals, macro) for a month plus, not just 2 weeks.