Extending aquarium for aquaponics

[David Palmer]

All my basil died due to the cold weather outside.  I'd grown accustomed to having fresh basil with my breakfast every morning.  After stumbling upon aquaponics, I'm now trying to extend my existing gold fish aquarium to include a grow bed for basil, maybe include cilantro or pak choi, etc.  I'm using spare parts I have collected around the house with the occasional trip to the hardware store.  I want to keep this inexpensive, but be able to grow basil.

I'm currently cycling the setup but I have a few concerns about the flood and drain schedule.  All I have on hand is a 30min timer.  I've set it to run the pump for 30 min, then drain for 1hr. The grow bed fills in under 5 minutes, then the overflow kicks in for the remaining time.  I have a valve on the drain so it drains slowly for the next hour.  Will basil be okay with 30min of flooding?

I also have a bunch of electronics sitting around; 555 timers, transistors, resistors, and capacitors.  Oh, and a bunch of LED's and a relay switch. So...

I was thinking that it'd be easy enough to put a couple screws through the side of the plastic grow bed.  This could provide a limit switch for when the grow bed is full to switch off the relay, thus turning off the pump.  I could set the 30min timer to alternate on/off every 30 min and run the pump for just as long as it takes to fill the grow bed.  This is closer to the 15min flood, 45 min drain schedule I've read about.

However, this is hardly running the full volume of water through the grow bed in an hour.  I have a 26gal tank and about 7gal grow bed.  The grow bed is full of lava rock, so estimate 40% of the volume is water, or about 3 gal.  I'd have to flood the system about 9 to 10 times to run the 26gal tank through the grow bed.  It seems odd to flood and drain every 6 minutes.  Is this advisable? 

Of course adding more grow beds would be good, but I don't have more shelf space.  Also, the aquarium only has 1 gold fish.  He's big (about 6 inches long head to fin) and will kill any smaller fish I add to the tank.  If I can find him a new home, I'm thinking a few small koi fish would be good and sell them when they get too big for the tank.  Mostly, I'd like to grow some basil before upgrading the setup significantly with new shelves and larger grow beds.

What would you suggest for getting this working?

[David Palmer]

It appears this is a common topic.  The following link to the Backyard Aquaponics trail is insightful,

Backyard Aquaponics trial

Initially, I figured that the primary issue in growing plants is with over-watering them.  The continuous-flow approach would cause the roots to rot.  Thus, either an auto-siphon or timed flood/drain was required

The evidence from the trial shows that this is indeed not the case.  Some plants do indeed suffer when their roots are wet all the time, so draining does help those plants.  But otherwise, the results showed that timed flood/drain performed best, then continuous flow, and in last was the auto-siphon flood/drain. 

But why?  It's not clear.  I'm now speculating that if the fish population were doubled or quadrupled, the timed flood/drain would result in the fish dieing.  Some evidence is present in that there were higher readings for ammonia and nitrites for this system.  Also, it only moves 1/4 of the water through the grow bed compared to the other two systems.  The good news is that this problem should easily be addressed by adding more grow beds, thus producing more vegetables.

However, I have limited space for grow beds.  This suggest either the auto-siphon or continuous flow methods are better as they can filter the water faster, thus keeping up with ammonia generated by higher fish density.  But which one would be better?  It'd be really interesting if someone would run the same trial with 4x the fish population without increasing the grow bed area and see how the three systems perform.

Given my resource constraints, I've adjusted my system to be a continuous flow for 30min or more with an occasional drain for 30min.  There is no need to drain for 1hr or more.  Doing so is detrimental as it would allow more ammonia to build up in the tank before filtering.  Going completely with continuous flow is probably a bad idea as I don't have the pipes to properly ensure an even flow across the grow bed.  The occasional drain should address this deficiency.

As far as worms go, someone commented that worms absorb air through their skin.  They can survive quite well completely submerged as long as the water is well oxygenated.  I already have an air pump oxygenating the water.

It seems like most any system works well as long as you don't stress it with too many fish.  Regular testing of the water is probably a good idea to get proper feedback about fish density vs. bio-filter capacity.

Do I basically have this right?

BTW, I measured how much water is pulled from the tank.  Out of a 16" high tank, the water level is brought down 2" to fill the grow bed.  This means that the grow bed is using 3.25 gallons of water.  The pump fills the grow bed in about 5 minutes.  Thus 30 minutes of pumping should run about 19.5 gallons of water through the grow bed.  I expect that this is close enough, so a 30 minute flood and 30 minute drain is a good balance.