it depends on a few factors..
based on what i could remember, the important ones are
bubble size - the smaller the better as there is more surface area between the bubble and the water to attract proteins and other harmful stuff
contact time - how long the bubbles remain in the chamber, thats y counter-current skimmers are better the co-current skimmer, even though they share the same height. some skimmers are designed such that the bubbles would swirl ard the chamber, increasing contact time.
how high the foam must rise - the higher the foam must rise, the 'dryer' the bubbles, the more black it gets, but not to the extent that the bubbles only break at the 'neck' and collects gunk there.
hth