Pool cleanliness comes home to haunt you when you get an infection. In regards to the visibility of a pool, the following information might be of interest. Please don't give me any excuses as to why an hotel in Bali might have lesser standards than those expected in Australia. Next time you book a spa, or stay in a villa with a spa, reflect on the standards in these regulations.
South Australia
Public and Environmental Health Regulations 1991
under the Public and Environmental Health Act 1987
6—Swimming pools
(1) Where a swimming pool is available for public use—
the pool must be fitted with—
(i) the filtration system that—
(C) maintains the water in a clean, clear condition so that a matt black disc, or a disc that contrasts with the colour of the bottom of the pool, 150 mm in diameter, is (or would be) clearly visible at the deepest part of the pool; and
7—Spa pools
Where a spa pool is available for public use—
(a) the water must be disinfected by chlorine, or by an alternative method specified by the South Australian Health Commission Code "Standard for the Operation of Swimming Pools and Spa Pools in South Australia"; and
(b) where chlorine is used to disinfect the water—
(i) the water must contain residual free chlorine to at least the level of 4mg/litre; and
(ii) the total chlorine concentration in the water, other than residual free chlorine, must not exceed 1mg/litre; and
(iii) cyanuric acid must not be used to stabilise the chlorine; and
(iv) the pH of the water must be maintained between 7.2 and 7.6; and
(v) the alkalinity of the water (calculated as a measure of the total amount of dissolved alkaline compounds in the water) must be maintained at a concentration of between 60mg/litre and 200mg/litre; and
(c) the pool must incorporate a weir off-take or skimmer system that continuously takes away surface water while the pool is in use; and
(d) the pool must be fitted with—
(i) a filtration system that—
(A) provides a continuous circulation of water through the filter; and
(B) passes all water in the pool through the filter at least once in every 30 minutes; and
(C) maintains the water in a clean, clear condition so that a matt black disc, or a disc that contrasts with the colour of the bottom of the pool, 150 mm in diameter, is (or would be) clearly visible at the deepest part of the pool when there is no turbulence in the pool; and
(ii) automatic equipment that continuously analyses and controls the level of disinfectant in the water, and the pH level of the water (at least to the standards required by this regulation) while the pool is in use; and
(e) the water must be replaced—
(i) at the rate of at least 20 per cent per day; or
(ii) at least once in every week by completely draining the pool; and
(f) the pool must be completely cleaned at least once in every week