I think the biggest concern people have with external overflows is just fear of the thought of having the overflow outside of the tank - what if the seal fails, what if the silicone doesn't hold and the overflow falls, etc.
Myself I wish I had gone the external route when I built my tank, but I didn't due to the above fears, being my first tank build I somewhat lacked confidence in my workmanship and design. But I will definitely be doing one on my next build.
They appear to work great. There's a couple ways you can put on an external overflow Reefer:
-Get one of the hang on externals (probably the most viable option for you since you have an established tank and want to add a sump)
-Drill holes in the back of the tank, install bulkheads with screens, install overflow box on back and have water enter it via the bulkheads
-Notch out the top of the tank to create a coast to coast weir that empties into the external overflow box (my favourite option and what I will be doing on my next tank. this is also the method that is implemented on pulpfiction's tank)
What method are you looking at implementing Jewel?
