There shouldn't be any options here. All reef aquariums
need to be open top. You need to enable your tank to 'breathe'. By not allowing your tank to breathe, you enable the buildup of carbon dioxide in the water, making it more acid. This lowers your pH.. and hell breaks loose!
Now I know some people have sumps that are open top.. but that is not enough. The surface area of this is very small and will not do much in balancing your pH and oxygen levels.