Backyard Aquaponics
A place to discuss aquaponics

I've seen recurring discussion of how anaerobic pockets in a growbed can be problematic for water quality.

What exactly is happens here — is there a particular by-product that causes fish health issues (say, hydrogen sulfide perhaps) or is it something more complex (e.g. bacterial toxins like how spinosad/botulism/tetanus work)?

Can I test my water to see if the levels of ________ are rising BEFORE they become lethal?

I am worried that a lack of reliable flood/drain may be related to my undiagnosed "floating fish" problem (see viewtopic.php?f=2&t=29409 for the full context) and also curious to experiment with denitrification. So it'd be nice to be able to either confirm and/or rule out this as an issue before I make it worse.

In a well balanced, sensibly stocked system it takes quite a while for anaerobic areas to develop, unless the system is really poorly designed. CF systems are more prone to developing anaerobic zones and sooner than F&D systems.

I have a system in my shop (siphoned F&D) that's been running 5 years and it was designed to turn anaerobic... Yes, you read that correctly. A few years ago there was a lot of discussion on the major forums re mechanical filtration, especially prior to the GB's, as well as system design, in regards to how long it would take to turn anaerobic. A prominent contributor on the major forums at the time and huge advocate of pre-GB mechanical filtration, decreed that the basic system design many people were running should actually be called the "sewerage system", and went further to state that systems designed in that way would not last 12 months before they become anaerobic and the fish and plants died from disease and rot... Many people had been running systems that way for years, so I designed a system exactly how he said not to, just to see how long it would actually take to turn anaerobic... it's still running fine today... falling apart, but still running fine... and at times it has been well overstocked (in winter when it's manageable).

In regards to detecting if a system is turning anaerobic... If your system has been running for some time, the pH has probably gone through it's natural decline and you would be regularly adding pH raising/buffering agents to keep it in balance. If this is the case and your system started to turn anaerobic, then the continual pH decline would ease, stop, and then the pH would start to climb on it's own. You may also start to see Ammonia and/or Nitrite readings, even in a lightly stocked system.

To answer your other question, it's the hydrogen sulphides that are an issue when the system turns anaerobic, especially if an anaerobic pocket is disturbed while there are fish still in the system. I've heard of fish dying within 20 mins in this situation.

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Mr Damage - a.k.a: Yabbies
Owner at Perth Aquaponics - Aquaponic Consultant & Trainer
Trade certified Horticulturist & Cert IV TAE

Hydrogen Sulfide is one of the products of anaerobic activity. Hydrogen Sulfide has a strong odor so your nose is a pretty good indicator of what's going on in your system. Unless you're stirring up the grow beds and they already have some anaerobic muck in them I doubt the fish deaths were caused by anaerobic pockets having formed.

I had a system with a completely clogged bed, sytem was about 10 years old, never had any fish problems..

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www.havehomewilltravel.com
Life on the road

Thanks for the feedback.

I do top off the system by dumping a 5 gallon bucket into the relatively small growbed. With all the roots in there I suppose nothing gets terribly stirred up (and aquarium doesn't get cloudy) but it definitely shuffles all the water around. I haven't noticed a particularly strong correlation between adding water and fish deaths — I've never had one die within a few hours of topping off and the fish generally seem excited to swim into the strong current coming out of the drain pipe during the top-off.

There's also no terribly off smell, just a bit of fishiness. (Come to think of it, I do have an old surplus store gas detector, so I should see if it has a working H2S sensor just to try it!)

I had four half-barrel pea gravel growbeds as part of my previous outdoor system that never cycled and didn't seem to have any trouble there myself either. That was a bigger system (so maybe not apples-to-apples) but that's why I hadn't been terribly concerned with fussing on the siphon.

So the verdict is against this being my problem. I'll try test for H2S above the air bubbler in the aquarium if I can, and keep this possibility in the back of my mind, but I guess I need to keep searching for a more likely culprit it sounds like?