We've used White King brand when we couldn't find Calgon. I think any brand will do the trick.
Dissolve two (2) cups of the water softener in a gallon of hot water. Then, pour the solution down the drain into the empty tank. Use two cups of softener for each wastewater tank in your RV. The tank's drain valve should be closed otherwise the softened water will just drain out. Then use the tank(s) normally until it is full and drain it normally. Add a cup of laundry detergent to the black (commode) tank at the same time you add water softener. This will help clean the tank. The gray water tanks should already contain soap through normal use.
Occasionally, I pour a half gallon of liquid bleach into each waste tank to sanitize and disinfect them. I add the bleach when the tank is about half full, and then continue to use the tank normally until it is full and ready to dump.
Don & Claudia
MOC 1685
1999 2850RK
1995 F250 Ext.Cab 4x4 7.3 PSD