QT all critters unless they are getting their own tank. ATM I have a designated QT. I haven't always, and will soon be selling this one off. In the future I will probably use a Sterlite tote with a few holes drilled in for the heater airline tubing, maybe a HOB filter, they have totes with clear to milky lids, a decent size and will hold lots of water.
Always I keep a filter pad or a couple bulk filter pd sized chunks behind rocks or plants in most tanks. As a double back up for illness in a tank, I can always use one from another tank that way. Also I will alwys use 2 filter pads in my HOB filters for extra BB surface and a ready extra in case I need it. I am down sizing numbers of tanks now but before I was increasing all the time so I always want an available filter pad.
Acclimate - put fish into decent size container - 5 gallon bucket (the bucket used for the dirty fish water, not for the new clean water) or gallon pitcher (think dollar store).
So say 1 gallon pitcher, make under half full of water add about 1/4 as much water from new tank they are going to go into. Then a cup of water till full about every 10-15-20 min (depends on fish and where they came from) South Denver has hard water, we have soft. I spent more time acclimating more slowly for something like that.
When the container is full I dump out (in the flowers or grass) about 3/4 of the water and continue same thing til full, dump out 3/4 of the water (to make it easier to catch) and then capture fish and add to the new tank. Never any water if I can help it in the tank from the fish store.
There are changes if there are delicate fish or shrimp or as mentioned they came from wildly different parameters. Or from a adoption from a person who never did maintenance, because a high amount of TDS can shock the fish going into cleaner water.
Then spray down the net and holding container, after rinsing well in tap water, with 90% alcohol let dry completely. Rinse before using gain.
Prime water conditioner too.
Almost always feed fish the praziquantel flake food after the first couple days of normal food, for a few days, then feed normally and then again for another day or 2.
Good idea for fish when the fish were wild caught or also for certin types of fish which have worms frequently, like Guppies and Angels.
Then normal variety of foods, bare bottom with a chunk of DW. Plastic plants and any additional live stems of what ever I have extra of, that if I had to throw them away I wouldn't be heart broken. Recently decided like the DW in the new QT tank for the tannins. So I might just grab a big ugly Mopani chunk and cut it in parts for seasoning the water in the future.
