Betta Fish are notorious jumpers, this is especially true of the fighter style plakat and HMPK. If you want to go sans lid on your Betta’s tank you will NEED to drop the water level significantly. Honestly its safer all around for you to keep a lid on your Betta tank, even a lowered water level may not prevent your fish from nose diving out of its tank. Halfmoon Betta are not as prone to jump out and generally cannot jump as far as a plakat but its still a risk for your watery friend that can be completely prevented with a lid.

So in short, yes, a lid is required to safely keep a Betta fish.

The reason a fish might leap out of its tank is not really well known. The common theory is that is unhappy with its current home either due to poor water condition, boredom, or size. You can reduce some of these issues, and theoretically their desire to kamikazi out of their tanks, by ensuring that your water condition is optimal, the fish has hiding spots, and that you are housing your fish in no less than 2.5 gallons. (We STRONGLY recommend 10 gallons minimally for long term housing).