I don’t want people to be scared away from this that is why you don’t need to acclimate just one or very expensive ones. You could just get feeder guppies as an experiment and you don’t have to wait 6 weeks you just need to find out whether or not you suppliers guppies have a reputation for being rather hardy. And if you really dout the guppies then just try out some mollies their more resilient and much larger and defensive than a guppy. Mollies being bulkier and most of them bred off shore, this means that they could have already been born in salt/ brackish water. And mollies aren’t an ugly option you can ask you LFS to get some really beautiful mollies such as sailfins, or golden mollies. I got unlucky that both my LFS don’t sell mollies. But yeah mollies their acclimation is as easy as it gets. Just drip acclimate them for a few hours until the salinity of your tank matches the water the mollies are in. Note that the acclimation should be done in a separate container, and they should be in the water they came in. But all in all mollies are the safest options for a brackish fish in a reef tank. And as a perk they’re algae eating machines. And if you have a large enough tank they can reach a max size of 6 inches. Although this is very rare it can happen. And if you get the sailfin variation they will show off all day long. Here is a screen shot of one I got from google.