Correct fallow period for ich is 76 days, not 8 weeks. You're just re-introducing a fish to a DT that still has ich.
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the ich organism cant come out of thin air...
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Funny enough, aerosol transmission is a thing, and has been documented at up to 10 feet if tanks are in the same room.
Only two ways to completely eradicate cryptocaryon irritans in a tank; fallow without a fish to host on for 76 days, or chemically with copper or chloroquine phosphate. Both chemical methods will kill all your corals, inverts and with CP, all algae.
To remove it in fish is either via tank-transfer methodology for 12 days to stay ahead of the reproductive cycle, or chemically with copper / chloroquine phosphate. Research papers have identified hypo-salinity resistant strains of ich, so that's really not an option anymore. Any "reef safe" treatments are just that, treatments. They will not completely remove ich.