Drilled Overflows Vs. Siphon Overflows

Drilled Overflows

Drilled overflows are standard with tanks that are sold as ‘reef ready’ aquariums.

With a pre-drilled tank, the overflow box is built into the tank.  The corner overflow is the most common but center overflows are sometimes incorporated into wider tanks since the overflow compartment can take up a fair bit of aquarium real estate.

At the bottom of the overflow are two holes that are drilled into the glass which is where the aquarium bulkheads and PVC plumbing pipes will be installed.  One bulkhead is for the water return to the sump and the other, for the return pump to deliver water back to the display tank.  Depending on the volume of the tank and the expected turnover of water entering the sump, each drilled hole can vary anywhere from 1 1/2″ to 2″.  The larger the hole, the lesser the likelihood of a snail or clump of stray algae blocking the overflow, preventing water from entering the aquarium sump and causing a flood!

Drilled overflows are good!

Siphon Box Overflows

If you buy a regular aquarium (read: meant for freshwater) without a drilled overflow, but you wish to incorporate a sump, one alternative would be to install a hang-on-back siphon box overflow.  (The other alternative would be to have the tank drilled and an overflow compartment installed)

The siphon box siphons water from the main tank and sends it via a tube to the sump.  At the same time, the return pump sends the water in the sump back up into the main display via another tube and the cycle continues.

Drilled Overflows Vs. Siphon Box Overflows

To put it bluntly, I would rather run a sumpless tank than to use a siphon box overflow!

The siphon box depends on an uninterrupted siphon from the main display tank.

Now imagine if the siphon were to stop.

The return pump will continue to pump water from the sump into the display tank.  But with no water returning to the sump from the display tank, the water would build up, causing the display tank to overflow, causing a flood.  Basically the entire volume of the sump will end up in the main display and eventually on the floor!

The siphon box is prone to failure, and it will usually fail when you’re away at work or on holiday.

Drilled overflows prevent Murphy’s Law from rearing its ugly head!

Links to more Saltwater Aquarium Setup pages:

Aquarium Tank Size | Length, Width And Height

The Ideal Location For Your Saltwater Aquarium

Ready-Made Aquariums

Aquarium Stands

Why Beginning Hobbyists Should Avoid Starting A Nano Aquarium

Aquarium Chillers

Aquarium Sump



Aquarium Covers | Put A Lid On It!

Cooling Fans


What You Don’t Need For Your Saltwater Aquarium!

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