The use of soaps, bubble bath additives, shampoos, conditioners etc in your whirlpool bath can all contribute to the build up of bath deposits inside the pipework. Substances such as Body oils, skin, soap scum, and general grime can be quite difficult to shift if it is allowed to build up over time. Any organic build up inside the pipelines can encourage the growth of bacteria, which  if not treated may result in an unhygienic system and ugly black flecks being seen in the water.
The conditions inside the whirlpool pipework are warm and damp, whch is the ideal environment to harbour bacteria. It is vital therefore that the pipework is flushed out regularly with specialist whirlpool cleaning and sanitising products in order to help reduce any build up of bath deposits and minimse the growth of bacteria.
All whirlpool bathtubs have a small amount of residual water left in the pipes, minerals in the water are capable of calcifying around the insides of the pipes. This hard scale build up not only restricts water flow, but becomes a breading ground for bacteria. As the algae breaks off, you will see it in your bath water as ugly black flecks.
An even greater health risk comes from the germs and bacteria that are left from previous bathers. Fecal matter, cold and flu germs are basic household germs that can be controlled if properly cleaned