Chlorine for your spa

To keep water clean in most soft sided hot tubs, a diluted form of chlorine should be used. (Sodium Dichlor).
There isn't nearly as much water as a pool, so you shouldn't need as much. About a tablespoon or two of granular chlorine once per week is sufficient.

Dichlor has a neutral ph and is also stabilized. It is the perfect sanitizer for hot tubs. Some people may argue that the stabilizer builds up over time, but if you drain your spa on a regular schedule it should not be an issue.

Here are a few other options.

Calcium Hypochlorite:  Cal-Hypo as we call it in the industry is loaded with calcium and has a high hH. This may help if you fill your tub with soft water, and it adds the calcium. Continuous use however will raise your pH swiftly and your calcium levels as well and could cause scaling on your hot tub walls and your equipment, mainly heater elements.  As an emergency sanitizer or one time use, it probably wont hurt.  It just shouldnt be the chlorine you use all the time.  If you do use it, you should test your water frequently and adjust for ph levels that will increase!

Sodium Hypochlorite:  Better known as "Bleach. It too can be used in emergencies but also has an extremely high pH which over time will cause scaling and possible staining.  Using Sodium Hypochlorite requires watching your pH levels closely and correction will be required.   

Tri Chlor:  Typically used in tabletized chlorine for swimming pools, tri chlor has an extremely LOW pH and a very high available chlorine content.  It will bring down your pH and Total alkalinity quickly and should not be used for hot tubs.