Hey,
Mix your salt in a Bucket(I like the 5gallon generic buckets with lids avaliable - most Hardware stores....Lowes, HomeDepot...etc)....PERIOD>. USe new buckets and not something in your Garage that you've used to wash your car.
STOP adding salt to the tank as that's one hellava bad habit to develop in this Hobby. You can kill all kinds of GOod stuff/bacteria if pure salt granules directly contact it.
Peace and good luck with the cure.