lets see
when I was running 10 gal no cover, never knew a betta to jump
when I was running my 20 gal with gsp's, no cover never knew a puffer to jump
30 gal no cover, never knew mollies to jump.
55g no official cover but the 4' lighting housing covers 60% of the tank top.
50g hex, has a has a cover
and my 125g has a canopy so it is covered.
but there is nothing wrong with keeping certain fish in a topless tank. It's just Oscars need one hence the cover on my 125g. jack dempsey's similarly, hence the cover on my 50 hex.