When u add water to salt you create super high levels as your adding your water. Now I know your adding the water quickly but it doesn't matter. You will get precipitation. Levels can go up swiftly but not down. The damage has been done.
The main reason for your spa death is the alk swing. If your old water cannot fill at least 75% of tank volume than you must make absolutely sure the parameters of new water match your old stuff. And I would still do a 2-3 hour acclimation in a good size container of the new mixture of new/old water. Even then without a very established bio filter , its a crap shoot.
The main reason for your spa death is the alk swing. If your old water cannot fill at least 75% of tank volume than you must make absolutely sure the parameters of new water match your old stuff. And I would still do a 2-3 hour acclimation in a good size container of the new mixture of new/old water. Even then without a very established bio filter , its a crap shoot.