New tanks - first fish lasted 1 hour

[Reefer-Noob]

New biocube 32. Fishless cycle completed. Ammonia 0/ Nitrite 0/ Nitrate 5-10 (10 on API, 5 on salifert)/ PH 8.0 /dKH 7.6/ Salinity 1.025.
Initially using tropic marin pro but that's become harder to get and my last water change (50%) was done with IO (not reef crystal).
Purchased a (super) small royal gramma online which arrived this am. Pretty quiet in the bag, hiding in the corner and not moving much but would swim a little when I slowly inverted the bag. This is my first online fish so I'm not sure how much action there should be. Followed the acclimatization process on that was recommended (floating bag, 1/2 cup every 4 In). After about an hour let the fish in and it promptly swam to a low flow corner, laid down and was breathing fast. Dead in 1 hour.

I'm kicking myself that I jumped the gun on ordering the fish with a bit of nitrate in the tank. Would a nitrate of 5ish (or the relatively low dKH) really kill a fish that quick? Did I acclimate too fast (I'm sure the dKH was higher in the bag)? I don't want to simply blame the company (the fish was still mostly alive when I opened the box and all). I'd love to hear thoughts on what anyone would do next. My current plan is to completely switch over to Red Sea Coral pro (and ultimately their system) and then try again when everything looks good (absolutely zero nitrate and a less borderline dKH).

Thanks!

My tank has 0/0/30 and I just finished cycling. I have a pair of clowns in there doing perfectly fine. I mean, I cycled the tank with them. IMO the fish probably would not have made it in anyone’s tank if it died that fast. Not like Royal Grammas are hard to keep. It probably came from the supplier weak or shipped poorly