[digital-audiophile]

My primary drain and standpipe are 1-1/2" diameter. I have a strainer on the main drain and the standpipe is just open PVC.

What happened was that I was having a hair algae bloom and in my infinite wisdom instead of pulling out my powerheads to properly clean them I figured that if I scrubbed them in place in the tank the GHA would simply just get sucked into the overflow and drain into the filter sock on the drain line in my sump, well instead of that happening as planned it gummed up the strainer to where the water was hardly getting through and raised the water level in the overflow box to where it had to drain into the standpipe.

Had I left this for a day without a standpipe I am confident that the water would have continued to rise to the point where it would have overflowed the tank.

I am glad though that I took Albert's advice and raised my drain/strainer a few inches in the overflow instead of just screwing the stariner into the bulkhead. Standing on a stepstool trying to reach down into the overflow with my return antisyphon in place was a PITA as it was, it would have been almost impossible doing it right down the the bulkhead without pulling apart my return plumbing.

Just food for thought, although this was a stupid mistake on my part and not something I would repeat the standpipe saved the potential of a flood.