Tanks with built-in overflows

[coco.puff]

Getting back into the hobby after 10 years and looking at tanks with built-in overflows. I noticed that most of the overflows have additional intake slots at the bottom and appear to be situated so low that they would be covered by a sand bed. Question: what purpose does this serve?

 

[Snoopy 67]

They pull in water from the bottom also.

 

[coco.puff]

Right. I get that. Just wondering why that would be necessary especially if it’s below the substrate level.

 

[pandaparties]

Right. I get that. Just wondering why that would be necessary especially if it’s below the substrate level.

Not everyone runs a sand bed, on top of that i've seen FW tanks run gravel a bit higher with a space underneath and have water pulled out from under - effectively vacuuming out underneath.

 

[coco.puff]

Not everyone runs a sand bed, on top of that i've seen FW tanks run gravel a bit higher with a space underneath and have water pulled out from under - effectively vacuuming out underneath

That makes perfect sense. I was thinking exclusively about marine tanks and didn’t really consider FW.