I’ve run man MANY longterm timelapse experiments. As suggested, mineral oil is a great tip, just be sure to layer enough on top so that the entire surface area is covered, in practice I find that warming the mineral oil a bit before hand also makes it easier when layering on top - avoid bubbles as they will mess-up your brightfield/phase/DIC. I used mineral oil from Sigma that is for mouse embryo work. Regarding medium “stability”, some people have luck adjusting buffering capacity by tweaking sodium bicarbonate or supplementation with HEPES (25mM final, pH 7.4), but with a good incubator this shouldn’t be much of a point. For some cell lines I’ve used CO2 independent medium and turned off the CO2, not all cell lines like the CO2 independent medium, it can be mixed with normal medium to ‘wean’ your cells onto it. I assume you’re system uses a modern autofocusing system that is independent of visible fluorescence, if not, be sure not to focus using fluorescent light. What do you mean by “medium stability”?