I have two tanks the order of fish were intended for. One has only been set up for a few weeks, but it was a 10-gallon upgrade from a 5.5-gallon in which I used all the same substrate, so I should have been good on bacteria. The other tank has been set up many years. I checked the water parameters of both tanks and they were good, no high nitrites or nitrates or ammonia. I didn't do drip acclimation, but put the fish in buckets with their shipping water and slowly added tank water, like put 1 cup in, wait 15 minutes, put more tank water, etc. over a couple of hours. I ended up only putting the cories in a tank because the other fish were too small to be put in with other fish and not be considered food, so I had to scramble to set up a small quarantine tank with a sponge filter taken out of the betta tank and sand from both tanks, for the bacteria present on the sponge and sand, and had to then acclimate the fish to the quarantine tank.
The place I ordered from is a small, family-owned business about 75 miles or so away called Aquatic Arts. Now that I think about it, I think may have made a mistake wanting to support a small, local business. Given how quickly they sell out after receiving new stock, I think the issue with this is that the fish are being doubly shocked in a short amount of time. They get shipped to this business, from who knows where, then they quickly turn around and ship the fish out to their customer. They probably get their fish from some place far away that actually does the tank raising like in Florida, California or maybe even Thailand for all I know. I'd probably be better off buying directly from someone who actually does the tank raising, even if it is far away, if they do 1-day shipping. Now I just need to figure out who that may be.
I usually buy my fish from PetSmart because that is the only place available nearby except for a garden center that also sells fish but always has dirty tanks. I had my heart set on panda cories, but PetSmart never has them. I've usually had pretty good luck with PetSmart fish and may just have to settle for what I can find there.
Sorry to be so long-winded.