This feels like an embarrassing question to ask, but are filter-cap tissue flasks (e.g. a T75) intended to have the cap/top tightened all the way on? In other words, will the filter membrane still allow proper gas exchange in a CO2 incubator? I've always left the caps somewhat loose without knowing for sure.
Yes, gas exchange works pretty well in totally closed flasks with filter caps. You only have to make sure that the cap filter is not dirty because of contact with media or something like that. If that happens accidentally, dry media can affect the ability of the filter to allow gas exchange.
Sometimes 'loosening the caps' will be written into protocols in our lab, but these protocols came from the days before filter caps were invented. Loosening the cap would be required because that would be the only way to achieve proper gas exchange.
Tighten the caps, and if they get wet from the inside or outside, make sure to change them out, as that can lead to contamination.
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