Let me put it this way, if you use a spa pool, if you can't see the floor or the jets in the wall, DONT use it!!!! Just think of it as if 20 people use it every hour for 5 hours you have 100 people who have used the spa, that's 100 people going in the same bath water as you!! But saying that, if the filtration plant is design correctly you have nothing to worry about, just think of the principle if the waters not clear (same for a pool) if it's cloudy don't use it. I wouldn't let my kids go in some pools while on holidays because of the water quality. Oh best time to use a pool etc in the morning