FWIW, I added a big ass UV filter to my system after losing 90% of the fish last year after the tank transfer and added new fish due to ich, and I haven't loss any fish due to ich. I don't believe in quarantine. To do a proper one, you need large space and resources. A little 20gallons is not going to cut it. I can't imagine what all the fish been through when they're stuck in this little glass box with all these foreign chemicals that suppress their appetite. A proper sized UV filter will kill most of the protozoa and give the fish a chance to fight the rest off the rest of it. I've not seen any adverse effect of adding UV filter on any of my corals. Just crystal clear water. The most important objective in keeping fish is to make sure they eat and eat well.