Kreem works well but be very sure the inside of the tank is spotless so the coating doesn't peel off the inside. It's a real PITA to get out and will clog everything. The kit comes with muratic acid to etch the inside of the tank but clean the tank to the best of your ability first.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!