Some people find it confusing when they receive their water kefir kit and their first ferment jar contains a rubber seal and a clasp. Should you attach the seal or not? Should you clasp the lid during the first ferment or not?
It really comes down to personal preference.
Water kefir can be fermented with either an open lid or a closed lid, or in other words, aerobically or anaerobically. Aerobic basically means a ferment with oxygen, which is done with an open lid. Anaerobic means without oxygen, which is done with a closed lid or airlock. Most people traditionally use an open lid with water kefir ferments. Either way works.
I suggest you don’t attach the rubber seal that comes in our jars. Just close the lid and clasp the hasp. That way air will get in and bugs will stay out. Fruit flies love water kefir so it’s best if you close the lid to keep them out. Water kefir should not build fizz in the first ferment. However, if you were to seal the jar tight it may build fizz. You may see tiny bubbles even if the lid is not sealed.