After doing alot of tanning recently i have noticed several things i was curious as to if they are intended or bug's.
You cannot fill an empty tanning tub with water directly, however you can add water IF tanning fluid is already inside the tub.
When the concentration falls below 4.6, the water graphic inside the tub vanishes, however you can continue to add water to fill the tub to full. (and get the concentration as low as 0.1)
Then once full you can bring the tanning tubs concentration up by applying brains to the tub.
Once the concentration is 4.6 or above the water graphic reapears and you are also able to start tanning with 4.6 concentration or above.