does condensation form on the underside of your lid?
depending on the temperature of my office, it can drastically change. At times it does create condensation on the underside of the lid, when the droplets join together and from larger and larger droplets sometimes my betta will jump at the water droplets.
I know that when i feed him, i use to put a couple of drops of water on the lid and soak a couple of pellets there so that they would expand prior to me putting them in his tank, he would hover under them and sometimes jump and hit the underside of the lid right were the pellets were.
just a thought