Glad that the tank temperature seems constant now, it's so important to the little guy's health!
Just another bit of info to add about tank lids - they're necessary not only to prevent the fish from jumping out, but also to maintain the betta's respiratory health. In their native habitats in Thailand, the air is warm and humid. A lid on a nice, heated tank maintains a layer of warm humid air at the water's surface, which is what the betta will be breathing. Their labyrinth organs are best adapted to air of this type, and if they are in an uncovered tank, the air at the surface will be much drier and colder, and this isn't very good for them.