There are many ways to sanitize your hot tub or spa, how do you decide which one? There is chlorine, bromine, alternative sanitizers, chlorine generators, bromine generators, shock and so on. All have their place and some are better than others for a small water volume spa with hot water.
Sodium Dichlor
If you are going to use chlorine in your spa, this would be the choice. It is almost pH neutral (will not effect the pH of your spa) and dissolves quickly. However, it does have stabilizer (cynuric acid) which you don’t want building up in your spa. If you drain your spa every 6 months then the cynuric acid buildup will likely not be a problem.
Trichlor
A great choice for pools, but for spas it disolves slowly, lowers the pH, has stabilizer, may cause bleaching of the shell if the trichlor tabls are in contact with the shell, and may leave a permanent water line ring.
Cal-hypo
Calcium hypochlorite is very popular for pools because it is relatively inexpensive. It includes calcium which can leave deposits on the spa equipment (heater and fittings) and can contribute to a calcium water line ring.
Liquid Chlorine
If you have it, use it but it raises the pH and is not the best choice for a spa with the water at higher temperatures.