Backyard Aquaponics
A place to discuss aquaponics

If they flood at different times as you said you could pull it off...
That should use up roughly 648L of your 1500 in your 18 GB's(852L remaining) - that should be deep enough for your fish as it is not even half...
BUT, if you use a sump and another pump to return the water to your tank it would mean more water out of your tank before it starts to return to your tank... (and I suppose we need to remember the water travelling in the various pipes as well)....

Pumping different beds at different times definately will help the fish tank to have a more consistant level and if set-up correctly could negate the need for a sump.

The problem being you need a pump for each bed or a valve that cam reliably switch between beds,,,,just one more idea amongst the millions:)

Here are the results of my experiment - rough as it was - no settling of media or roots present...
THe info below was taken from my thread here

I think this may be a good location for this question.

How high up should the water come on my GB? I kinda have the impression that you want it to flood all the way and leave only about an inch or so of media above the water line. Or do you not fill it up quite so much? Now if i have a 100L GB and a 50L fish tank, it should only take a few liters to fill the 100L GB because of my media?

Sorry if this is in the wrong section.

Thanks, Jason

Correct, when flooding your grow bed, we usually say to flood to about an inch below the surface of the gravel or other media. That is just a standard flood level based on an average assortment of plants in a 12" deep grow bed that is flood and drain. You generally don't want the surface of your media to get wet as that would cause algae to grow (and use up nutrients you want for your plants.)

Now there are varriations on the rule. Some people will adjust the depth of their flood while they start new seeds so that the seeds near the surface get wetted and can germinate. If you are growing plants that are susceptible to crown rot you might want them in a grow bed that doesn't flood as near the surface. For example, a citrus tree you might put in an extra deep growbed that leaves the top 4 inches of the growbed media dry since citrus is prone to trunk and root rot if the surface media or trunk are constantly wet. Strawberries are another plant that might enjoy special treatment (say towers) to keep their crowns dry and keep the berries from touching the media where they would tend to rot.

Now don't let this discussion on water displacement confuse you about the ratio's when people talk about grow bed to fish tank ratio (like 2:1 grow bed to fish tank) they are talking about the total volume of the grow bed container compared to the total volume of the fish tank container. Figuring out the displacement is only about knowing how much the fish tank water level will fluctuate or knowing how big of a sump tank is needed to handle the flood and drain operations.

It is hard to tell. I use clay, gravel, river rock and a mix of them. The last lot of gravel was finer and takes a lot less to fill. Also after time with solids and worms the ratio would change. Maybe on the next GB I will fill and measure and repeat at intervals later to get an idea.

Well, if the surface area of your grow bed and your fish tank are the same and your grow bed is 12” deep, the water level of your fish tank will rise and fall approx. 5” as the grow bed is flooded and drained. Actually closer to 4 1/2" and that is with no plant growth yet.