" water softener pellets"
This is your problem.You want regular water softener that is used for laundry
I use the cheapest I can find and if it is a powder, I dissolve it before dumping down the toilet. Usually a cup of softener and whatever water it takes to dissolve it and then add a cup of cheap liquid dish soap. That's it, no bleach or anything else. Now add 5-10 gallons of water and go for a drive. I treat after I dump and then drive to the next spot. Longer drive the better. Now dump again and flush the tank. If you are really full of it, you might have to repeat a couple of times.