can you fill your water level of a tank all the way up?

I fill a few mm higher than the bottom of the plastic framing (just enough so I dont see the water line). This leaves me about 1" to the top or maybe a tad more.
 
Guess my initial understanding is not really correct then. Because i remember reading those power filter ads saying something like "support massive biological filter with fully oxygenated water passing through the bio media etc etc" so i assumed the water is fully oxygenated, then passes through the filter's bio media which makes the bacterial happy(they love oxygen!), then that water is put back into the tank, rinse repeat times infinity.

but guess in reality the water the filter sucks in are already oxygenated to begin with (from surface movements)? So the process of moving through the hob's 3 stage filter really doesnt add much oxygen to it.
 
i i fill mine about 3/4 inchbelow the top
my tank doesnt have any frame..so i need to keep it kind of low to prevent water from spilling.
 
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