Good timing. I just wrote this up and posted it on another site:
I have fielded this question many times and answered it verbally, but finally got around to making a few sketches.
The question is, if I feed my scrubber directly from the overflow, how do I ensure that I don't cause my tank to overflow if the slot pipe clogs?
If you are running your scrubber from the siphon line of a BeanAnimal 3-pipe system, you already have a 2nd and 3rd pipe, and you have no worries, and can ignore this post. If you are running it from a Herbie, you may or may not be able to do this due to the inclusion of a siphon break, but your secondary line is your backup (if appropriately sized).
The first thing to address here is that I personally have never heard of anyone having this issue due to algae growing into the slot, assuming their slot is cut to the appropriate width (an overly narrow slot will inherently restrict flow). The reason is that as algae grows into the slot, water backs up in the pipe causing head pressure. This pressure forces water through the slot harder, which inhibits algae from growing further into the slot. Basically it's like a built-in control system. SM described that to me early on in my scrubbing days and it is 100% true.
The real concern is when something makes it's way down the overflow and blocks the slot. If your screen is inserted into the slot such that it touches in inside of the top edge of the pipe, you create a divider in the pipe upon which something like a snail or anemone (or small dead fish) could potentially block the flow. If the screen is only inserted such that it sticks up a little bit, whatever finds it's way to the slot pipe might get pushed to the end, but could still partially block flow. Either way, unlike a standard overflow drain pipe, you have a restricted outlet instead of an open pipe. Not that an open pipe cannot clog either - any overflow system is prone to Murphy's Law.
Here are 2 sketches that show the overflow bypass method of helping to prevent disaster:
The first is for a sump-based setup (shown with a bottom-drain / drilled tank)
Second is for a remote sump (in this instance, a basement)
The horizontal pipe should be as low as possible. The standpipe should extend up as high as possible, then elbow over to a tee which goes up a size or two. That tee is open on top as an anti-siphon break (not really necessary now that I think about it, but I'm not going to re-do the drawing now!!) and then the bypass standpipe extends to the sump.
The concept here is to allow the head pressure to build up. If you were to simple put a tee in the horizontal line with the side outlet pointing up and then put in a double-elbow to route the water down, that setup will quickly start to flow water as algae grows into the slot, and as there is no head pressure to prevent further growth, you will bypass the scrubber completely in no time.
The external standpipe allows for full blockage of the slot pipe while maintaining the overflow rate from the tank, preventing the tank from overflowing. The catch is that when the bypass is operating (slot clogged), the maximum flow rate of the system is limited by the level difference between the maximum tank water level and the high point level of the bypass elbow. This flow rate can be calculated using
BeanAnimal's Hydraulics for the Aquarist Calculator
I should note that I have not personally tried this, so there may be a need to keep the elbow-to-tee transition a little lower that you think for an additional safety factor. The biggest thing to remember is that your bypass flow rate will be reduced, and you should test the system by shutting off the flow to the scrubber to simulate a full blockage and verify that your tank doesn't overflow.
