Sadly, IME, even with a cover, if they want to jump, they will!
In nature if they jump, they can jump back into the water...with these aquariums, they are lucky if my cats don't eat them[my cats bit off my Paradise Fish's tail after he jumped!]. lol
Someone in another forum blames me for not having enough current, but I don't think that was the case. I think the betta scared the sh*t out of him, but he didn't know he scared the betta quite bad. Still, the betta misses his friend and is hanging out, sad, by the filter.