The thing about corals, in a lot of stores therese very often fish in the frag tanks. Some fish are what Ive always called "ick bombs", Powder blue tangs in particular. Then clowns. Ive left some really nice corals at the store because they had just introduced a PBT or other ick bomb into the coral tank. I avoided clowns for three years btw when I started in the hobby.
This is one more reason to qt coral, the others reasons are nuisance algaes and unwanted hitchhikers.
Many tanks run with ick and fish parasites. Also many run with unwanted hitchhikers as well, Zoa eating nudibranchs and flatworms, some nuisance algaes ,ie all pests, are the easiest to explain.
Somehow in the tank a balance has been reached to keep the population of the pest under control. But if these are introduced into a system without the checks and balances of the last tank(something that eats them), the populations "bloom". Similar to the Pod population bloom you recently had.
Also don't feel too bad as even with full Prophylactic treatment fish still may pass away mysteriously weeks even months later. It easily could have been an injury sustained from bagging at the store. Yes, I finally one day told a guy I want buying the fish because it was wrestled out of the tank at the store. So imagine how it was put in.
I learned a lot about Quarantine and conditioning of fish from Bob Fenner and PaulB, as well as Sprung and Delbeek, still internet but not rumors, myth, opinion and facebook. Many many of those named in "conditioning" will also repeat the need for good food as nutrition is VERY important to an animal's immune system. So studying more for long term success with theses animals is pretty important. Just like ones first puppy chicken or Emu, there's going to be a learning curve. And for many its this continuing learning and study that actually is part of the attraction(addiction), to the hobby. For some that don't want to go that deep, a basic understanding and general protocol or method works extremely well. Theres great info here on R2R thanks to Humble and Mel. Def well worth reading.
Heres Mel's Awesome video On freshwater dips.
https://www.reef2reef.com/ams/video-how-to-do-a-freshwater-dip.214/
