Supposedly the gas that is used will not diffuse into the water. I have had a house tented with a FW tank and I can tell you that everything pulled through just fine. I cannot comment on if the gas would effect a reef/SW tank at all.
The biggest thing I would be concerned about is heat as the electricity much be turned off, meaning no A/C in Miami will translate into a 90+ degree house. I believe we did it in the winter. Hopefully somebody who needed to tent a house with a SW tank will drop by and comment.