My well water is high in iron, nitrate, silicates, and phosphate. I believe the caulerpa takes care of the iron, Nitrate & phosphate and I believe the snails take care of the diatoms caused by the Silicate.
If you look closely at my picture you'll see two buckets. The fist bucket is the mixing bucket and it has a small vent hole to prevent a vacuum. Not a lot of air passes throuh it but some does. It's the second bucket that I have the lid tight on. As the vacuum builds in the second bucket it just sucks limewater from the first bucket.
If you only have one bucket you'll need a vent hole. This hole won't prevent CO2 from entering but it will slow it down.