Roy,
I had a similar question, but using water from a creek.
If you do install your project, then you must have a filtering system in place for 100% of the water that is transferred from the upper pond to the lower pond.
You could install a smaller pipe (like 2") to move some water from the upper pond to the lower. Set that pipe between the full pool level and the normal pool level, that way you only transfer excess water from significant rain events and cannot drain down your upper pond.
This pipe must have a filter at all times! It also must fail in "safe mode". For example, the filter clogs and quits transferring water, rather than the filter clogs and is blown off the pipe allowing unrestricted water to the lower pond.
Bill Cody says the mesh size to capture all fish eggs and fry is 500 micrometers = 0.5mm.
FireIsHot posted in this thread how he uses his filters.
https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=527981#Post527981Another alternative would be to capture water from significant rain events ABOVE your upstream pond. (Assuming this is only from grasslands or forest with no other ponds or seasonal streams.)
The additional piping required would only be a little more than the length of your upper pond.
Good luck on your new project!
