
OMG, your poor fish.
The biggest problem here is that 7.5 ph is NOT high enough for you to be messing with.
When you did your big change to try to get the salt out were you VERY careful to get the temperature the same as the tank water?
Did you use dechlorinater?
The huge ph change from your RO water to your tap water is VERY stressful.
Most fish can adapt to your home ph levels. I would stop trying to change it, that just puts more of your $$$ in the fish store pocket, with lots of room for error, as happened today.
It does sound like your fish are having problems, but at this point there really isn't much you can do. Tomorrow am I would do another 25% change.
Your tank cycle needs close monitoring. Please post tomorrow what your test levels are.
Ammonia?
Nitrite?
Nitrate?
ph
By the way, I would take the jug back to the store and let them know what happened, so they can hopefully help their employee know not to do it again in the future.