There are a number of hot tub retailers who claim that their product requires no chemicals.
The HSE (Health & Safety Executive) and BISHTA (British Irish Spa & Hot Tub Association) confirm that an oxidizing sanitizer like chlorine must be used to a value of 2-5ppm to maintain safe water. Bathing in water that does not have a safe level of sanitizer may result in illness.
Ionizers, which utilise silver or copper to inhibit bacteria growth and ozone generators, which oxidise bacteria, are excellent secondary safety systems. On their own however they are insufficient to maintain safe clear water.
There are a number of products that claim one cup per week will keep your water safe. Owing to the complex nature of water and the varying use patterns of hot tub owners, it is impossible that a one-cup sanitizer can maintain its strength, unchecked, over a seven-day period.
Regardless of make HSE and BISHTA recommend the use of ionizers and ozone systems, which are backed-up by an oxidising sanitizer, which is tested regularly.
Please do not be duped by hot tub retailers claiming "too good to be true" water care products as they do not meet HSE and BISHTA guidelines and far worse may put your family's health at risk.