Do you think it might be possible to have a viable system that has the beds flooded every 8 hours, by hand? I'm imagining a bed lowered with a car jack to a level below that of the fish tank, thereby flooding the bed. Then, the bed is hand-raised by the jack back above the fish tank level. This is done every 8 hours, which would allow for 3 times a day (not too inconvenient). To keep the bed from drying out too much, those round balls in seed starter soil could be added, as they act as slow-release sponges. And, the fish tank could be conical in shape, so that the poop collects at the bottom so that when the bed is flooded it will be sure to get all of the poop at once. Could a balance of fish density and plant density be obtained to create a balanced system, and not require a pump? Thanks for your opinions on this.
in a situation like that you would want to have as natural a fish tank/pond as possible so that there is a lot of surface area for bacteria and plants and algae to act as a first-phase filter.
you'd also want a fairly large volume of water for the comparatively low-stocking density of fish so that during that 8 hours ammonia is not able to reach toxic levels.
personally, i think you might be better off leaving the pond and beds alone and instead manually pump water into a 'header' tank that would slowly drain into the fish tank and overflow into your beds carrying solids with it. just be sure to setup the beds so that they can drain into a sump to allow you to reuse that water when you manually pump it back to the header tank. this would be easier and give you more options of where to position things without compromising the plant beds. it's also going to be easier to use existing hand pumps commonly found in third world countries to move your water rather than inventing a system of platforms and levers and gears.
have you given any thought to making it like a teter toter and use block and tackle to lift and lower each side. you could use valves to control the flow through the hoses. the reason this seems more feasable because if this is for third world applications, one would have to keep in mind the avalibility of matirials.
Thanks again for all your insights. Robert, even easier than teeter totter would be just raise/lower only one side with block/tackle, namely the grow bed. Yes, there would be hand-operated valves between bed and pond, and probably two hoses, one wide for the flooding, and the other narrow for the slow drain. Ellen, yes in the heat of the day probably a shade cloth would be needed. It sounds like from your above responses that it might be plausible, which is encouraging. Proof is in the pudding, of course! Thanks, all.
Just a thought Eric... why so keen to raise and lower the grow bed? It is likely to be very heavy, and is going to have to be extremely sturdily made (which could be a problem) in order to put up with constant lifting. Surely it is much easier to raise the water using whatever methods are locally (and more commonly) available? The grow bed could be partly over any pond area as that would help with shading... and any leaks would just go back to the pond. Just seems like lifting the grow bed is a lot of hard work and likely to break fairly quickly.
I suspect that under most circumstances a permaculture type of arrangement is likely to be more effective than an aquaponics one. Take a look at the work of Bill Mollinson - he has done a huge amount in developing sustainable farming practices in 3rd world countries. As much as I love aquaponics, it is more likely that permaculture will provide a more effective and stable answer... IMHO.
have you thought of using treddle pumps? very low cost low tech option. especially for third world or off grid applications