Internal vs external over flows

Tradam Yaks · Aug 18, 2026

Is the main difference between internal and external overflows whether the tank is drilled on the bottom vs the side? Not including non drilled tanks with siphon overflows or otherwise. Meaning a box on the outside would be needed for a side drilled tanks therefore being external as a bottom drilled which can be plumbed into a sump directly. I suppose one could direct plumb a side drilled tank too.

If so, are there pros and cons of each aside from having plumbing, boxes, and bulkheads visible on the outside/inside of the tank? I understand having something on the outside can limit placement like being flush against a wall but more curious about performance than esthetics. I feel like having external offers more options. Like I can set up a herbie or bean animal easily.

For whatever reason im having a hard time finding these answers.

Insight into my experience. 20 years ago I had a reef tank for 3 years. 75 gal display coral tank with sump, plumbed with a 30 gal refugium, and a 29 gal tank display with other fish I didnt want in the reef. I ran a siphoned overflow and had all these tanks connected. None of the tanks were drilled.

exnisstech · Aug 19, 2026

Internal allows the plumbing to pass into the stand with no plumbing outside behind the stand. It's a cleaner look and will allow the tank to be placed closer to a wall. They both work the same IME. The biggest negative of an internal for me is the loss of real estate inside the tank and having to scape around the overflow. I have a peninsula with an internal but it is the entire back wall on a short end of course so I only loose about 4" of total length. On a lagoon often its dead center and sticks out. I really don't know if there is an actually difference in the amount of flow possible between the two if it were measured. I've not noticed any functional difference between them. I suppose an ext could be a possible failure points for leaks.
PS I agree that there are more options for customization on an external.

Tradam Yaks · Aug 19, 2026

exnisstech said:
Internal allows the plumbing to pass into the stand with no plumbing outside behind the stand. It's a cleaner look and will allow the tank to be placed closer to a wall. They both work the same IME. The biggest negative of an internal for me is the loss of real estate inside the tank and having to scape around the overflow. I have a peninsula with an internal but it is the entire back wall on a short end of course so I only loose about 4" of total length. On a lagoon often its dead center and sticks out. I really don't know if there is an actually difference in the amount of flow possible between the two if it were measured. I've not noticed any functional difference between them. I suppose an ext could be a possible failure points for leaks.
PS I agree that there are more options for customization on an external.

Thank you for the response. Clarified this for me well.

mfinn · Aug 19, 2026

As already stated, they pretty much act the same.
But in my last tank build I decided to go with a 2 part external overflow box with the main part on the back wall of the tank with a 3 drain pipe system.
My main reason was reclaiming space on the inside of the tank.
You do need to have the stand and tank further from the wall, but a few years ago I found one of my tanks was damaging the wall with salt spray on the sheet rock with the back filters being so close.
Maybe that was just a me thing and relative to anything, but it made me think.

BigMoe · Aug 19, 2026

Been debating this for my next tank. Internal is a cleaner look. But you do lose lots of real estate in the tank. I'm please with hob drains so far. May get next tank with corner drains.