Thanks
Most of us do not have the space for a proper quarantine system. Or the discipline to follow through it.
My process to manage fish disease :
Quarantine. I have a plastic tub (filled with 20L of water) to quarantine fish for a couple of days to monitor them. Usually add prazi as a preventive measure against intestinal worms , skin flukes & paravortex turbellaria flatworms (black ich). But as you say, this is not 100% fool proof. The rest are more applicable to your query.
Hospital tank/medication. I have a spare plastic tub / tank on standby in case fish get sick & needs to be taken out to medicate. Air pump & wavemaker for the hospital tank is always within reach. Standby Seachem Cupramine (for ich), Eiho Prazi Gold (dewormer) & API Furan2 (broad spectrum anti biotic)
AquaUV sterilizer. Usually running about 75% of the time but will turn on 24/7 for several weeks if new fish are added. UV is to mitigate risk of full blown ich. I'm running a 57w unit on the 850L tank.
Feed the fish well so they can fight off parasites on their own. I use Fauna Marin Food Energizer on a regular basis to give pellet food a nutrition boost. Selcon works very well as well.
Cleaner wrasse eat some turbellaria flatworms that reside on fish. Wrasses & Mandarin dragonets eat what they find on the sand bed. Wrasses & mandarins are not a solution to pests. And may not do much in the whole scheme of things. But they probably do help a little (just like how UV can reduce the population of cryptocaryon irritans). A single adult turbellaria flatworms (black ich) can re-produce hundreds of larvae on the body of a fish. Any reduction of an adult is a good thing. I have cleaner wrasse, 6 line wrasse & blue mandarin dragonets - 2 pair of each.