I have used the 7 6 5 3 and one gallon carboy. They all work well. I have not had any issues with the screw on tops and or the O-Rings. The plastic is somewhat thin and if you pick them up it squeezes in the sides a little bit. This can cause air movement into airlock. However if you do not overfill them it will not put any wine into the airlock. Overall excellent product but would like to see them a little thicker.

1. The description says it's made from heavy PET but the wall thickness is actually quite thin. The problem this causes is that when handling it once it's full of liquid the sides push in decreasing the volume of air inside. This causes pumping of the liquid in the airlock causing concern that the airlock's function could be temporarily compromised. 2. The o-ring doesn't stay in the groove of the lid so that putting on the lid requires that the o-ring be carefully placed on the rim of the fermenter and then the lid even more carefully screwed on so as not to disturb the location of the o-ring until the lid is tight.