Ok I hope im doing this right.....

you probably have a bag good for 75 gal. or so, unless you have a 5 gal bucket water change mix.

 

since you have no corals or livestock, i would start that tank over personally. nuke the whole thing and redo. sounds like the person who owned the tank previously didnt have the time to care for it and when he told you everything in there is dead, that would put up some warning signs in my head to not keep anything he had going.

bleach rock clean the sand and add newly mixed saltwater and let it cycle for several weeks.

 

that may not be a bad idea, running it in low salinity for a few days may kill everything too

 

are you putting sea salt you cook with directly into the tank ?