Aquarium overflow vs bottom of tank intake

Question

Wouldn't something that sucks out detritus from the bottom be better than something that can only pull floating stuff?

Reasoning:
A wet/dry filter will usually feature an overflow, whereas other types of filters (hang on back, canister, etc..) suck water out of the bottom of the tank via an intake. I'm assuming that the intake will filter out more solids than an overflow.

Now I understand that a wet/dry system would detox the water better, but you'd end up with detoxified water with a lot of particulates.

The articles I'm reading indicates that a wet/dry system does a better job of keeping your tank clean/clear, which to me is counter intuitive.

Answer 1

For saltwater tanks it's easy: there is a lot of current in the water. If there would be any detritus, in would already be floating and it would be sucked up by the overflow. When you look at saltwater tanks, nothing is floating. So there isn't any real debris to be sucked up by a filter inlet deeper in the tank.

A surface skimmer or overflow filter will remove what is one of the things that do need to be removed: proteins. Foam will be produced at the surface and that will get sucked up by the overflow. Anything in the water column itself isn't a real problem and if it's small (nitrate, nitrite, ...) it will be floating in the water and it will also be sucked up by an overflow filter.

A wet/dry system also filters water just like other filters would (you can put any filter media in) but as an added bonus, it also skims the water surface.

In freshwater tanks, filter inlets are typically placed deeper in the tank because floating debris is more common due to less current.

You also rarely need a surface skimmer in a freshwater tank because they are only used to remove oily film and that's a sign of lack of maintenance.

Under gravel filters (what is typically called a bottom filter but I know you don't mean this) is a completely different story as they will remove debris but also a lot of nutrients that plants need.

Answer 2

A lot of your finer particles that need removed will float in saltwater, with the proper flow will go into the overflow. Also, said system will have a cleanup crew (crabs, pods, worms) to eat the particles that don't make it to the surface. That's what they are for. If the system contains coral, it should also have a heavy, random flow. That would assist in suspending particles to get to the overflow where they can be filtered out. Overflows also assist in adding oxygen to the water column by the flow itself.

You don't mention if this system has a substrate. That may affect where you draw water from. Especially depending on the size of the substrate particles.

If you have something drawing water from the bottom of the water column only, the surface of the water is not getting cleaned. Also, I would think there should be some sort of filter in front of the intake to prevent fish and what-not from going down the drain, which would create something else to maintain.

Many tanks built for saltwater have a layer in front of the overflow itself with additional intakes at the bottom to help draw from the entire water column.

Detoxifying the water can be done many ways. Wet/dry, sump with a skimmer and mechanical filters, bio-balls, live rock, GFO, carbon, etc. It all depends on what's in the tank. If the tank has SPS, it's critical PO4 is kept at a lower level than what would be required with fish only. If the tank has a lot of large fish, such as tangs, poop removal is most critical since they are essentially nitrate factories (unless you like algae).

This article discusses surface layers and agitation and why it improves the health of your aquarium: http://www.advancedaquarist.com/2009/6/beginner