What’s the difference between regular instant ocean and the blue box?
One is more expensive than the other.
Everyone has their reasons for buying one vs the other, it's all the same in my opinion. And sure, if someone wanted to they could dissect my words and argue that it's not all the same, I have no interest in that kind of debate though.
My corals don't seem to care either, and that is reason enough for me to pick the cheapest and run with it. There isn't anything in the more expensive salt mix that's not in IO, both will mix to natural sea water parameters and support life, what more does one want? It's been claimed that residue from IO left in the mixing container is clay binders, but until someone from IO publishes something about it, that's all just a claim some guy on the internet made up. Salt mixes are likely all fundamentally equivalent, why would IO need a clay binder? What is that even? It's salt crystals that are milled, some salt mixes are milled finer than others and probably done so to dissolve faster, that process probably adds to the cost. I'm certain there is a sweet spot regarding particle size that the manufacturer shoots for when milling, if it's too fine it will compact in the bucket and if it's too coarse it take longer to dissolve, somewhere in the middle is probably the best size. Like others have said I don't get residue in my brute from using IO, I do get some precipitation in the form of scale but it's not enough to even raise my eyebrow over, it's certainly not an OMG I need to change salts thing. I'm mixing 25 gallons today I'll post a photo of my brute immediately after I empty it.
I've read years ago, even before the internet, that certain elements in salt mix are present only because of impurities in the raw materials. So I dunno which salt is the best, perhaps the best salt is the one with the best printing on the bucket, even that adds a few bucks to the cost. So start to add up all of the things that a particular salt says it includes or has more of, and there you have your cost differences. At the end of the day it is my position as an advanced hobbyist to say it really doesn't matter what salt you use, pick the one that works for your budget, or is easy to get, or mixes clean, or what your favorite LFS sells so you can support them, or what your favorite influencer on YT uses, etc. I do know for certain that if you have a successful tank growing corals that it is not always a good idea to switch salts cold turkey, if you have that plan in mind, start blending in new salt slowly over several weeks/months.
I didn't post this with the intention of creating a thread ending conclusion, I think about a lot of things behind the scenes but I don't always take part in discussions or offer up a post, even though I may have something constructive to add
