I understand the cost involved in a proper QT set up, that is why there is a market for this product, as well as "reef safe tonics". Trouble is, none are as effective as proper quarantine.
Safety Stop is formalin in the first dip and methylene blue in the second. Formalin dips can work with Brooklynella and flukes, but formalin does not work for Amyloodinium and Cryptocaryon, it just doesn't kill all of the trophonts. I'm not sure why they bother with methylene blue, it is a really outdated medication. It is sort of bacteriostatic, but it isn't an antibiotic. Be sure to aerate the formalin dip!
Never use Safety Stop on shipped fish, the formalin is too stressful after a long journey in high ammonia water. The reason I know this? Years ago, I had come up with this really cool idea; a triage treatment essentially the same as Safety Stop - I dipped all shipped in fish and then moved them into a quarantine system. My losses were outstandingly high, and I stopped after a couple of shipments. I now let shipped fish recover for 72 hours before hitting them with any meds. Fish from your local LSS, with shorter bag times, don't have the same issue.
Jay