Reddit mentions of Lifegard Aquatics 1/2-Inch Standard Threaded Bulkhead

What you want is a Bulkhead Fitting. It makes a hole on a flat surface and allows you to attach PVC parts to it. I would say you add this right under the lip of the top pond and put ~1' length of PVC pipe on it. The water will fill to the pipe then drain out into the pond below. The only real consideration is the strength of the plastic and the length of the pipe. You wouldn't want to crack the pond.

Hey friend. you're on the right track with your idea to drill a hole and plumb it to a larger tank. Be careful when you choose the spot to drill. You don't want to drill in to a very round side, as that will be harder to make watertight. i would recommend trying to find a nice flat spot, just below the max fill of that unit's tank, and then drilling a hole for a bulkhead fitting(so that its watertight) and then just attach a hose to the drain end of the fitting and you can just set the other end of the hose in to a 5 gallon bucket below the unit or something.


a 1/2 bulkhead fitting is less than 5$ on amazon(also might not hurt to look around for a smaller sized one of these, as a 1 1/2" hole may be kind of big for this small tank) i only looked around for a second.
https://www.amazon.com/Lifegard-Aquatics-Standard-Threaded-Bulkhead/dp/B0002Z7U1K/ref=sr_1_15?keywords=bulkhead+drain+with+gasket&qid=1565284477&s=gateway&sr=8-15


If you go with this route, i would get a step bit (drill bit for drilling a precise hole by gradually widening it)
https://www.amazon.com/Knoweasy-Sizes-Titanium-Drill-Drilling/dp/B07NKXLTCB/ref=sr_1_1_sspa?keywords=step+bit&qid=1565284634&s=gateway&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyMEU0WExMM0Y1NUNZJmVuY3J5cHRlZElkPUEwMDQzOTM5RjNOQkZRRFgzUVFXJmVuY3J5cHRlZEFkSWQ9QTAwODExNDgyUEtUR1lBS1NJNzM4JndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==


then you would just need a female 1/2 thread to 1/4" barbed fitting and some 1/4 tubing to run to the bucket. I have similar setups to this for top-off tanks in my hydro systems. If you need any help figuring it out and sourcing parts, just shoot me a message!


Best of luck!!

Then why aren't you just using a bulkhead fitting? Something like this:
https://www.amazon.com/Lifegard-Aquatics-Standard-Threaded-Bulkhead/dp/B0002Z7U1K/ref=lp_700755011_1_6?s=industrial&ie=UTF8&qid=1565222743&sr=1-6

with a threaded plug should be reasonably waterproof.