Saltwater acclimated mollys and an occasional royal gramma or clown fish are my way of watching for illness and keeping the tank "alive". And the occasional bad tempered fish " cough cough Half black angel" that isn't trust worthy. I operate a MARS 200 gallon system like the ones at Walmart n pet co have as my QT for fish. I medicate only as needed w/Chelated copper, i run slightly HYPO salinity which usually match the shipping water closer, and find moderate UV and reduced heat " 75-76" average flow reasonable water chemistry to be very effective with incoming fish. I always wait at least 45-60 days in QT before adding them to my main tanks. This gives me plenty of time to fatten them up and monitor behavior and so forth. I keep inverts and corals in a separate QT set up with much higher quality water n chemistry standards.
The hardest thing for me to understand was how to balance the first 40 minutes of a new fish arriving to your home. If a fish has been shipped in a bag with oxygen then things happen when you first open the bag that begin the ammonia cycle which is bad. But also because the fish has been in the bag the Ph will be low and possible the oxygen as well. if you dump them in new water they get a ph spike. if you add an air stone they get an ammonia spike. if you leave them in about 2/3 the water they shipped in add a 1/5th of new water immediately and then begin a drip acclimation i seem to think thats the best of all the options and reduces stress adjusts Ph and temperature and stave off rising Ammonia most effectively. I usually add a few pieces of PVC pipe from my existing system to add hiding places and immediately begin nitrogen cycle. And use the drip acclimating time to check the salinity is matching up and the PH, temp and look for signs of immediate issues. I will often try to feed them a few things "fish eggs, brine shrimp, etc" in the bucket just before I move them to the tank by hand transfer to not scratch up the skin with the nets too much and then dump everything after as waste.
I do think at certain times of the year or with certain fish a fresh water dip would be a good idea if the fish seems stable and calm and ready to deal with that beefier they go into your QT. I find that i almost never do it though unless i really think something is on them and don't want it in the QT anymore than absolutely necessary. Of course 200$$ plus fish also get even more careful treatment and the process is slowed and they usually ship in a much bigger bag than say a CHROMIS would. Not that they don't both deserve good care but honestly the clock starts ticking when you open that bag! Many folks just dump em in and say the stress of all the other steps negate the value of them on many species. Hardy fish is another topic. And there is defiantly more stress for a fish from a bag to a bucket acclimating, Vs one dumped into the tank and able to just immediately hide and do its thing to get adjusted. You can not do this with inverts they will just die is the opposite end of the spectrum and my inverts spend hours acclimating instead of 40 minutes type thing.
Also the difference you keep your QT water at Vs the fish was in before shipping and then shipped in also adds alot to all this. If your gonna get a fish from a reef system Vs a wholesaler, the salinity will be much different.
I also try n keep lights low. If a fish looks reall bad in a bag then the odds are not good when they get out but some fish like wrasses play the drama queen thing. lots to be learned about fish and I don't medicate with anything till there in the main QT and swimming and showing signs of food interest if possible. Don't add medications to the acclimation ticket unless you sure how it will effect the water chemistry. many of the various treatments reduce oxygen rapidly and effect PH which is already unstable during the acclamation process.
Im not an expert and i don't fully understand what happens to fish " when the bag opens" I hope others can help with more information. Good luck.
I would say I am down to 10-20% losses from shipping. Being they arrived alive till 48 hours after introduction to QT to be honest, when it used to be 40% dead in 48 hours sometimes at worst when I started and didn't understand all the factors. The store is more than happy to sell you a second fish of course so educating your self here is well worth it.
