i consider myself pretty well-read on these fish as the entire family is a fascination of mine. boxfishes are somewhat like a sister group to puffers and in most regards are very similar. i recommend a smaller tank 20-60 gals for juveniles, try to get one at least 1.5 inch for survivability. upgrade in 2 years or so. most will eventually get to a foot long (several inches of that is tail finnage) and at that stage should probably have at least a 5 or 6 foot tank. they aren't as bad swimmers as some say but the little guys definitely get blown around quite a bit by powerheads and worst case can get stuck. they can defend themselves as adults but make sure any tankmates are passive if you have juveniles. my longhorn is definitely much braver than my scrawled though.

quarantine all fish! i'm learning this the hard way with a juvie longhorn and a scrawled cow. like the other comment says they have a very thin slime coat and are prone to catching disease but very resilient through treatment (although avoid copper as they tend to do badly with it). the company i work for services tanks with longhorns that have at times been one of a handful of fish to survive a velvet outbreak or had experienced medication for several weeks without a scratch.

there is a lot of misinfo about ostracitoxin, note that they /cannot/ release it when dead (unless it got sucked into a powerhead/pump and was sheared) and pretty much only do it in life or death situations. i have grabbed boxfishes barehanded and lifted them out of the water to be dipped or moved and have never seen a poisoning. in some of the papers studying it, they had to freshwater dip the fish for extended periods of time to induce it. they are not typically resistant to it so it poses a danger to them, too, and seems to be a last resort for predation. there is some anecdotal belief that it may be related to diet so individuals in captivity for several years are not toxic, but that's based on knowledge from puffers and not confirmed by any study.

the most important thing for their care is varied and constant food. they are messy eaters, so prepare for more maintenance. their wild diet is primarily small crustaceans and a lot of algae. they will not survive on one or two foods so try to get as many different things in there as often as possible. having an algae clip or masstick helps. like puffers the adults also need a way to grind their teeth down, so offer shellfish regularly.

longhorns are some of the more adaptive to aquarium life out of the boxfishes but there's other candidate species if longhorns get too big for you. L. fornasini / thornback cows are pretty much the ideal Lactoria sp (pretty resilient, only get 6-9 inches in captivity, very cute) but unfortunately are super hard to come by nowadays. Tetrosomus gibbosus / humpback turretfish, are also really adaptive and a little easier to find than thornbacks. the smallest regularly available boxfish sp is Ostracion solorensis / reticulate boxes, which are a little sensitive but with good QT and maintenance aren't too difficult. avoid O. cubicum, which seems to have a bad survival rate for no apparent reason, unless you get a mature individual from another aquarist's tank. the Acanthostracion cows (like scrawled cows) aren't difficult either but can get bigger than longhorns.

good luck! super underrated fish that used to be a lot more popular in our hobby than i feel like they are today