not a check valve... a cut-off valve... for airline tubing pressure adjustment. much like a gang valve but one input and one output.
I have been using check-valves between the bubble counter bottle and aquarium feed and also a check valve between each yeast bottle and the bubble counter. They just automatically cut off when I remove an individual bottle and if you squeeze the yeast bottle it prevents back flow into either the bubble counter bottle or the squeezed yeast bottle.
Ultimately I chose the check valve solution because I feared I would forget to re-open the the cutoff valve and lead to an exploding yeast bottle. Yeast dripping all over the family room would not conducive to a harmonious marriage. It protects me from my forgetful tendencies.
Dundadundun is correct in that cut-off valves enable you to completely and securely shut off any portion of the circuit. Cut-off valves are also a about 1/2 the price of check-valves as well - so that is a bonus.