Just got home from vacation and found a dead fish I was only gone for two days

1cup of salt

5tsp

it works also u can go to pet stores(if they are any good) and get amonia treatment

A fully cycled (=healthy) tank should never have detectable levels of ammonia. Ammonia of even 1ppm can prove lethal to fish over time. Your tank is not cycled or you lost a significant portion of your beneficial bacteria colony (which, in significant numbers instantly process/eliminate ammonia in the tank).

Second problem: Major pH swing.

pH 6.0 is TEN TIMES more acidic than pH of 7.0. That alone provides enough stress to kill a fish (matters made even worse with the presence of ammonia in the tank). Going from 7.0 to 6.0 over a weekend indicates poor water quality (your carbonates are being depleted and when that happens pH drops).

Solution: Conduct regular water changes (probably 25% every other day for a week, and thereafter 25% weekly) . Salt and additives won't really work at this stage. You need to establish adequate water parameters first. Once that is done, fish will be able to thrive in the tank.