The MythCommon knowledge in the reefing world states that a one inch line will flow 600 gph gravity fed. This is just not true; this number is double what one inch lines will actually flow safely. There is the important qualifier: safely!
Well, half of 600 will be 300 gph. Yes, this is really all it will flow without other problems. These problems include siphoning, the water column building up above the inlet of the overflow and noise. In reality these problems are related, all are linked to flowing more than the drain line(s) can handle.
Siphoning is perhaps the biggest of the issues. In fact many of the overflows utilizing 1' lines rely on siphoning to achieve the advertised numbers. The siphoning can occur as a full siphon (no air introduced into the line in any way) or a more common partial siphon where the line is aspirated (air introduced), but not to the point to completely prevent siphoning from occurring. The problem with siphoning is it cannot be relied upon to happen. If a tank is running a 1' drain at 500 gph and the power goes out, it may take several minutes, if at all, for the siphon needed to drain this flow to start. What happens in the meantime? You guessed it, water on the floor! The same scenario plays out should there be even the slightest obstruction on the line, the siphoning allows for no safety margin at all.
The second issue, the water column creeping above the drain inlet is just as dangerous. As the water flow through the line exceeds the line's capacity the water level will raise above the inlet of the drain. As this happens the pressure above the inlet will increase the flow. When the pump starts up the water will climb, then drain back down, up, down until it reaches equilibrium at some point. Siphoning will usually occur in this case also. The danger is twofold: the water may indeed climb high enough to overflow the tank before this equilibrium is met. Imagine your sump return turning on just to watch the water rise to the rim of the tank and over the top! The second, this is a flow balancing act, one little thing changes (and it will) and you again have water where it does not belong. This situation also falls victim to the slightest obstruction on the line.
The third problem that occurs above 300 gph with a 1' line is noise. I cannot tell you how many hours I have spent working out noise issues with people running too much flow through their overflow drains, 1' line or not. This issue too generally comes back to siphoning...notice a trend? The line in full siphon mode can drain 5-6 times what it can truly gravity fed. So what happens? As the flow exceeds what the line can handle the water rises above the drain inlet. Then in a good world a siphon starts. The siphon quickly drains the tank's water level back down to the inlet of the drain line. Then air is introduced, breaking the siphon, water starts to rise again and the cycle just repeats'¦. until the one time the siphon does not restart! Any of us that have spent time around various overflows have heard the cyclic flushing/slurping noise associated with this. The fix? Flow what the drain can truly handle gravity fed without siphoning, 300 gph in the case of a 1' line!
So what do we do about it?
Either drill or have drilled bigger holes in your overflow dang it! I hear from and troubleshoot for way too many people that have bought either 'reef ready' tanks or hang on the back overflows that are just disappointed with the flow. There is no way around it, physics is physics. Larger or more numerous drains are needed. If you think about this before buying or setting up a tank you are ahead of the game! Drilling your own holes is one option. It is fairly easy, straightforward and through the last few years the diamond holesaws needed have gotten dirt-cheap. Another option is to custom order a tank with larger or more holes drilled per your spec. Any good LFS can help you with this, if they won't find somebody who will. The manufacturer will usually charge X amount per hole, well worth the cost. As for how big, how many, I feel the industry standard should be 1.5' lines, not 1'. A 1.5' line requires a slightly larger hole and will easily safely flow 750 gph plus! Jump that line to 2' and you are looking at more than 1300 gph without issues.
So what is a 1' drain good for?
Nanos, pure and simple. I'm not talking a 40 gallon nano either, more like the 10-15 gallon type! Anything larger has the space and deserves a larger drain.
Testing the things! Anybody with a container of known volume (actually measure the thing out yourself) and a timer can test these flows. Time vs. volume moved is easy enough to measure. I have personally spent many (if not hundreds) of hours actually testing such things. A 1' overflow with a vertical 24' line and no siphoning will consistently yield right around 330 gph. Add an elbow, horizontal run, strainer (God forbid) or any other plumbing and 300 gph quickly becomes generous.
One last word to the wise. A 1' drain on a 90 gallon tank can provide enough flow for filtration with 300 gph. Another whole issue is overflow redundancy. That is, if the drain fails/gets clogged, is there another to take up the slack? Overflows should always come in twos, double what is needed!
My question I have asked myself for years is 'Why do we even have to deal with this?'! Well perhaps it is pure laziness of manufactures not testing their product or flat out misleading customers. The fact of it all is we need and should have larger throughputs on our overflows. Do not be satisfied with 1', bigger is better here!