Four gallons. I pull the baby plant out and add nutes then bring it to 5.8 and check again and it's up to 7. Something in four or so hours, this happened for a week and the plant looked poor, I had lots of bubbles and a scum ring around the top as well as the air stones are slimy. I put them in soil and and hoping they will come back. I bought a dark blue bucket from Lowe's and a proper growing lid. It's my first dwc and it's something I would like to be successful in.