This was very informative. Thx bro. I'll definitely will be quarantining and treating from now on
Don't lose hope, I have also lost a bunch of fish etc. I'm coming up to 12m on my tank.
Yeah they go into QT/HT beyond what I could do atm but after I kept watching QT related videos I understand why it's the best thing to do, at least do the minimum so you can look back and know you did your best.
I was lucky so far, no 'visible' signs of diseases in my DT atm, *touch wood*, I might have ich but it's probably not enough overwhelm my 2 clowns, 2 damselfish, the one time i didnt research the fish its aggressive >: and a blue assessor.
For catching fish btw, you should have at least 2 nets and/or i used a brand new, rinsed in rodi water washing machine bra net thing with the zipper to trap the fish when i had issues with 1 of my damselfish and no lights, apparently red light is good, but with both the fish do realise eventually what you are upto and hide =.=
I lost a bunch of corals (impatient and using brilliant, and H2O2 [30%] can be over done easily.... ).
Inverts and fish: 1 hermit crab, 4-6 snails, a shrimp, a white goby, sand sifting goby (yellow head), six line wrasse, 1 other goby I forgot the name, 1 anemone and a bristle tang (dipped the tang and seemed okay for i think about a week and suddenly died.
Deaths were H2O2 inverts cannot handle it neither can corals. Fish were random, e.g the white goby suicided into my power head, watched it happen

i used onion nets to encase were the water jets out of the powerheads and around were it pulls in the water, so the blue assessor cant get pulled or the anemone, but i'll leave it because i plan to get a anemone again one day.
After losing the tang, I will now QT every fish, as I want my current fish to live a happy n healthy life

For corals bayer dip :| (especially for SPS I use my current DT water mixed with bayer, had SPS and didnt think about the harshness of the RODI, of course disposing the old bayer water)