Use an oil skimmer to remove any oil film. If you don't want to do that, you can also wick any film or light growth off the top of the coolant, using paper towel......like Bounty, which is a good one.
I actually 'skim the skimmer tank' this way: lay sheets of Bounty on top and let it wick up the flotsam with its natural absorbency. Use a new dry sheet for each pass until it looks like the film is gone. I then squeeze the fluid out of the wet towel and use it in a squeeze bottle on the saw or the lathe, where it just seems to disappear forever . The wet towel can be laid out to dry. Incinerate the dirtiest looking sheets, but the cleaner looking sheets can be saved for the next time to scavange the grungiest flotsam.
I use Hocut 795B for synthetic concentrate and RO water. This seems to reject bacterial growth very well, if one takes care of the surface of the coolant. I get 1 to 2 years use out of a batch of coolant. Sometimes it may sit for a week or two with no disturbance except for the skimmer.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)