Backyard Aquaponics
A place to discuss aquaponics

For constant flood you set your outlet at your high water level. This can be done with an outlet through the side of the growbed or a standpipe through the bottom with the top of the standpipe at your high water mark. I would use the standpipe option because you can adjust the water level by changing the length of the standpipe. This also gives you the option of making the bed flood and drain by placing a capped pipe over the standpipe and creating a bell siphon. You would set your water level 1 inch below the top of your grow media. By keeping the top of the media dry you discourage algae growth on the top of the growbed. There is a thread on the forum that discusses which plants do better in constant flood and which prefer flood and drain.

Yes normal standpipe with a 7mm hole drilled near the bottom for draining from top and bottom of bed.

Going to throw a spanner in the works for you Davo

average grow bed is only filled to 0.3m deep
Your 1900 ltr tank is 0.5m Deep
If you filled it to 0.3m deep only then its 1449 ltr tank
add this to your 350 ltr tank and you end up with 1799 ltr
account for gravel / media in the bed @ 50%
your sump on the listed below system needs to be 899.5 ltr or over
not wanting to step on toes here but as your system seems to be progressing it sounds like its going to be a plumbing nightmare and the one below would actually work.
Would have to keep a close eye on the water level with not much headroom but still.

Also you list the 1/2 IBC grow beds at being 500 ltr
the total volume of a IBC is 1000L and tanking into account your only using 2/3 of 1 IBC to make 2 grow beds they are more like 300 - 350 ltr