I've built several aquaponics systems in my backyard (currently around 800 gallons worth) and have helped build out several others for other people.
I have some constructive criticism about these designs.
1) Sand does not allow water to flow quickly enough and makes it more difficult for oxygen to get to the roots. You might end up with anaerobic zones in the grow bed. Use a grow media that is bigger (pebbles and/or hydroton are good options). Make sure the pebbles are not sharp though, you want smooth or rounded. Your hands and your grow bed liner will thank you.
2) Timing circuitry adds complexity and another point of failure, just make it run continuously. Pumps for a backyard scale aquaponics system don't consume that much electricity.
3) Depending on stocking density you may want to aerate the water so your fish don't die. Air stones work, but running the pump continuously and agitating the surface of the water will do far more.
4) Don't put the pump in the fish tank. Ideally use a sump tank that the water overflows from the fish tank into a sump tank that is lower. If you have to put the pump in the fish tank, make sure it is not at the bottom of the tank, put it about 1/4 to 1/2 way down the tank. That way if a tube springs a leak, or if the grow beds don't drain and overflow, or any other unforeseen failure comes up, you don't pump all the water out of your fish tank and kill them.
5) No need for PEX. Just stick with simple 1/2" PVC. They are cheaper and more common. Especially the fittings. PVC is perfectly safe for aquaponics. People bring up safety concerns but they are unfounded. The concerns for PVC safety are when using high pressure or hot water (neither of which is a concern with aquaponics).
6) Read up on fishless cycling to prepare the water before putting in the fish. If you use ammonia make sure it is just ammonia (no bleach or perfumes). You can speed up the cycling if you get something that has the nitrifying bacterial already. You can buy solutions that contain bacteria or just get it from an existing system / pond.
7) Get the pH dialed in before adding the fish. The nitrifying bacteria like a pH of around 8 for optimal growth but once they are establish, move the pH to the ideal balance for your plants / fish. The nitrifying bacteria will grow fine at any pH once fish are introduced so setting the pH to 8 while cycling is really only an optimization to speed up bacteria colonization.
8) Once fish are added, move the pH very slowly (ideally only 0.2 pH per day). Sudden pH changes are harmful for your fish.
Thanks for these points, bcheung! Here are my thoughts on each point:
1) The sand-based system is based on Dr. McMurtry's aquaponics research at NC State University. Here are a list of publications about it: http://iavs.info/publications/ In general, coarse sand is a great substrate for plants. Both research and my own experience show no problems with aeration or anaerobic zones forming when set up correctly.
2) The timers are really no more complex than the pumps, so I really don't see them as adding that much complexity.
3) The way the water drains through the sand means it keeps dripping and agitating the surface for several hours after the pump stops pumping water to the vegetable beds. Of course, if you plan on maxing out the stocking density, you should add bubblers.
4) Because the water starts trickling through the sand seconds after the pump starts, you only have ~10-15% difference in water height.
5) If you want to eat the fish, might as well try to eliminate anything that might get into the fish through the plastic. It doesn't add that much cost.
6) That's a good idea, but adds the requirement of finding pure ammonia available for sale.
7 and 8) I think most people will just add filtered tap water and go with whatever pH that is.
I have some constructive criticism about these designs.
1) Sand does not allow water to flow quickly enough and makes it more difficult for oxygen to get to the roots. You might end up with anaerobic zones in the grow bed. Use a grow media that is bigger (pebbles and/or hydroton are good options). Make sure the pebbles are not sharp though, you want smooth or rounded. Your hands and your grow bed liner will thank you.
2) Timing circuitry adds complexity and another point of failure, just make it run continuously. Pumps for a backyard scale aquaponics system don't consume that much electricity.
3) Depending on stocking density you may want to aerate the water so your fish don't die. Air stones work, but running the pump continuously and agitating the surface of the water will do far more.
4) Don't put the pump in the fish tank. Ideally use a sump tank that the water overflows from the fish tank into a sump tank that is lower. If you have to put the pump in the fish tank, make sure it is not at the bottom of the tank, put it about 1/4 to 1/2 way down the tank. That way if a tube springs a leak, or if the grow beds don't drain and overflow, or any other unforeseen failure comes up, you don't pump all the water out of your fish tank and kill them.
5) No need for PEX. Just stick with simple 1/2" PVC. They are cheaper and more common. Especially the fittings. PVC is perfectly safe for aquaponics. People bring up safety concerns but they are unfounded. The concerns for PVC safety are when using high pressure or hot water (neither of which is a concern with aquaponics).
6) Read up on fishless cycling to prepare the water before putting in the fish. If you use ammonia make sure it is just ammonia (no bleach or perfumes). You can speed up the cycling if you get something that has the nitrifying bacterial already. You can buy solutions that contain bacteria or just get it from an existing system / pond.
7) Get the pH dialed in before adding the fish. The nitrifying bacteria like a pH of around 8 for optimal growth but once they are establish, move the pH to the ideal balance for your plants / fish. The nitrifying bacteria will grow fine at any pH once fish are introduced so setting the pH to 8 while cycling is really only an optimization to speed up bacteria colonization.
8) Once fish are added, move the pH very slowly (ideally only 0.2 pH per day). Sudden pH changes are harmful for your fish.