Curious - I had a similar experience. Bought an ostensibly healthy mag from a local reefer. Maybe 1/2 an hour drive, and ended up having to treat it with Cipro otherwise I would have lost it. Perhaps damage from being moved, stress, who knows - very sensitive animals.
I'd echo OrionN about lack of protocol. I first heard of using antibiotics on mags from the chap who used to run the NYC Aquarium - maybe 1995? No specifics though.
I too had an experience with a gigantea where I picked it up, and after a 30 minute ride home, and by the time I opened the bag, its mesenterial filaments were hanging out. However, I don't think this in itself would cause it to die, as once it settled into its QT tank, the filaments retreated and the nem expanded greatly. I think it's a combination of the stress of travel, as well as the ailment already present within the nem. I actually felt that my gig was healthy enough to enter the DT after no deflation for two weeks. However, once in the DT it quickly slid downhill -- I am now down two gigantea in 3 weeks. I am now wondering if there was something still present in my DT water that caused the downhill spiral of my second gig.
What's interesting to note is that these nems reacted to Cipro and Septra in different ways, but the end result was the same -- and unique in how the nems physically looked -- both became extremely bleached and began eating themselves and the manner in which they did so happened very quickly (over the course of a couple of days). This leads me to believe that they suffered from the same ailment, but that I received both at different stages of the infection.
I also agree with Orion that many people have experimented with antibiotics but were tight-lipped about the process. I think the key driver -- unfortunately -- was finding the cure to ailments that would allow for a monopoly of sorts when it came to the sale of healthy gigs. What we know now is that there isn't the so-called "magic cure" (yet -- and I remain optimistic) and I believe it's because there are multiple ailments. I agree that collection points/export/import play a key role, as I suspect that gigs recently purchased probably came from the same source. I know that three of the four that I know of all died, and we used the same Cipro protocol that proved successful in the past.
In hindsight, I realize that there were crucial mistakes that I made:
1. Do not introduce an otherwise healthy gig into an environment where a gig died a week prior (we'll need to figure out a safe schedule, if there is one). I wonder if the ailment is caused by an agent like ich where it goes through various cycles.
2. Do not introduce a bleached gig to a DT unless it is showing a good feeding response while in QT. I think my clowns were rough on the nem, more so when I attempted to feed it.
Definitely a tough loss. Until we learn more, I don't think we can say we have a "cure" (not that we ever said that, but I think some were learned towards it) because I strongly feel like we've encountered something new with this crop of gigantea.