There are so many options how to flood and drain a media-based aquaponic system. I thought I'd mention a few of the options that have tempted me.
Timer-based flood. The idea here is that you flood your growbeds for 15 minutes each hour (a standpipe in each bed prevents the beds from overflowing). When the pump turns off,the water drains back into the fish tank. Pure simplicity.
15 minute timers can be found at Home Depot for $15. You have to find a pump that will push the full volume of your tank out to the growbeds in 1/4 of an hour. For my case, with a 100 gallon tank, that means I'd necessarily need a 500 gph pump.
A downside is you are draining your tank by whatever amount of water fits between the media in your beds. In my case, it seems a 50 gallon growbed takes ~10 gallons of water to the top of the standpipe. Even if I had four 50 gallon growbeds hooked up to my system, I'm only sucking out 40% of the water volume. So I'm not draining the tank dry, but it will be more crowded during those fifteen minutes.
Constant Height of Pond (CHOP). With a CHOP system, you let the growbeds drain into a sump tank. A single pump in the sump tank pumps water to the fish tank, which then "overflows" into the growbeds. The growbeds use bell siphons to ensure the beds both flood and drain. The fish tank is always at the same level, minimizing stress on the fish.
Australia-based Murray Hallam announced an update to the CHOP system in October 2010, where the water from the sump is pumped out to both the fish tank and the grow beds. The fish tank overflows to the sump tank. This system yields all kinds of benefits, including the ability to have beds at different heights and the ability to add constant flood beds. If I had more space, I'd be loving on this as an option. Alas, I'm stuck in my tiny area.
Murray Hallam's New CHOP2 System
Modular Fish Tanks. One reason I chose not to go with the CHOP2 idea is the problem of getting a 200+ gallon tall fish tank into a basement room. However, San Diego-based Tilapia Mama has a kit that converts four 55 gallon drums into a 200 gallon "Backyard Fish Farm (BFF)." If your space allows, you can use one of these BFF units as the tall fish tank for a CHOP 2 system, and you can lower your growbeds from the 48" top height I've got in my standard 3x5 system.
Indexing Valves. These are so cool. Each time you turn off the water, the valve switches to a new port. These were just invented a few years ago and are sold by K-rain and Fimco for irrigation.
TCLynx' animation of an Indexing Valve
Florida-based TCLynx of Aquaponics Lynx has pioneered the use of indexing valves for aquaponics. She modifies the basic indexing valve with a "gravity stem" so it can be used in low pressure systems, and is the only source I know of for the modified valves appropriate for aquaponics. The animation below shows an indexing valve being used to sequentially irrigate multiple growbeds from a single pump and fish tank.
Four growbeds flooded using a single pump and indexing valve
In order to use one of these indexing valves, you have to have something that stops the flow for a minute or more before starting the flow up again. While you could use the 15 minute timer, a repeat timer will allow you to shut the flow off for just a minute (I found cheap repeat timers on eBay for $30).
The modified indexing valve doesn't require the pressure you'd use in an irrigation system, but it still needs something like a 1000 gph pump to get the valve to work "right," according to TCLynx - a great option if you are looking at a 300-600 gallon fish tank.
What I Plan to Do. I love the indexing valve. I'd honestly love to use it.
I only have space for four 50 gallon grow beds, five max. I'm probably going to make do with two 100 gallon fish tanks, connected with piping so they stay at the same level. A 250 gallon pump would cycle all the water each hour, though I'll probably bump it to 350 or 500 to make sure there's enough flow to run the bell siphons. That will be only 6-11 watts per growbed to irrigate each growbed.
Right now I can't justify the added power, complexity or space to do CHOP, CHOP2, or use the indexing valve that's sitting on my desk. And since I've got a bell siphon design that works like a champ, I have no reason to opt for the timer option.
So it's simple bell siphons for me - at least until something changes to make more advanced technology/complications the obvious solution...