I'd say a conventional hinged glass cover.Define cover? Egg crate is a cover but allows good gas exchange. Glass is a cover and depending how its used can seal a tank or if gapped allow gas exchange. Consesus is a sealed sw tank is very bad
Do these allow all light from aquarium fixtures to pass through? I'll have to check out my local Home Depot then.Light diffuser. Its the plastic little squares.home depot lowes ect would have it