Today successful reef tanks employ a 10-20 GPH turnover rate.  Turnover rate is the number of times your total tank gallons "Overflow" from your tank to the sump & returned to the main tank per hour.
Ex.:  75 gallon tank x 10 per hour (Min) = 750 GPH (gallons per hour) turnover rate.

There are 3 basic overflow methods used in the hobby today.  They are:

A.  Predrilled or "Reef Ready" Pictured here an All Glass overflow/return unit. Permanent placement, low GPH, restricts internal water flow, restricts return placement & takes up valuable tank space	B  Custom Drilled Drilled w/bulkheads Permanent placement & GPH, water height low inside tank, normally mid intake screen	C.  Overflow Box Sets Hang-on tank rim (Sm box on R is inside tank) Flexible placement anywhere & anytime change is needed.  Custom GPH, just add more units to increase GPH or remove as needed. Simply The Best!

A:  PreDrilled Tanks:  All Glass Flow Rate GPH & Placement


The diagram above is looking from the tank top down.  The red arrows indicate water flow.  Using All Glass as an example I found that the vast majority of their Predrilled tanks were WAY under the recommended minimum 10x GPH, allowing only 600 GPH maximum per overflow.  Outflow & return placement is permanent and take up valuable tank space completely from tank top to bottom allowing for flow "dead spots".  It is critical in this hobby to have outflow & return placement flexibility as successful tanks have corals that grow & flow needs to be adjusted at times to accommodate tank changes.  These built-ins take up valuable inside floor space as noted by the red arrows in the diagram.
Lastly, the holes/slits accommodating overflow are very small & are easily clogged by coraline & detritus.

Pros:  Never experiences a siphon break.

B:  Drilled Tanks:
A better option than Predrilled but again, no placement flexibility, outflow placement is permanent & GPH is final once up & running.  Also, water levels tend to be lower than other options as the water will spill out at the base of the hole.

Pros:  Will always outflow as long as intake screens are clear.  Do not take up valuable tank space nor restrict internal water flow. Returns can be placed as desired & somewhat flexible.

C.  Overflow Box Sets - Simply The Best!
I recommend using them on tanks 30g & bigger.  These provide the highest GPH per overflow without sacrificing water your level.  Does not take up valuable tank space nor create dead spots.  These units can be placed anywhere on your tank rim allowing complete outflow & return flexibility.  Not a permanent fixture so can be used on tank upgrades etc.  I've used all methods & by far prefer these box sets...some of which have been running at my facility over 10 years without any trouble!

Con of the Overflow Box Sets:
Air getting trapped in the top of the "U" tube & eventually breaking the siphon if left unattended.

Why does this happen?  Our fault!  Not enough flow running through the "U" tube to keep air bubbles from getting trapped.  When water flows through the "U" tube fast enough the sheer velocity of the flow will ZIP any bubbles completely through the unit.  Run too little water through & bubbles can start to collect.  Let me be clear here...it takes an extremely slow flow to collect enough bubbles to break the "U" tube's siphon!  If this happens, I dare say you are running too few GPH in your tank to begin with!  In any event, you can see well in advance if air is starting to build up at the top of the "U" tube & remedy it before it becomes an issue.  Again, these units have been running at my facility over 10 years without any trouble!
 

If you have questions or comments regarding this topic feel free to e-mail Penny.

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