Backyard Aquaponics
A place to discuss aquaponics

Doubt it

The biofilter would only be converting ammonia and nitrite to nitrates (unless its anoxic). Normal fish tank (with biological filtration) will build up nitrates in the water.
The GB will just provide extra area for bacteria (and a support for your plants)

strange, let us know the results

If you have anaerobic sections of the filter, that well may be it. An unintentional RSG filter, as it were?

Tony,

Speaking as newbie....

Seems to me that, as others have pointed out, your nutrients are probably just too darned low. Only plants (inc algae) will remove nitrates, so low nitrates=too few fish. I figure I have about 250g of fish in my system and a handful of wheat sprouting in the window-box grow bed make my nitrates plummet in a week. A couple weeks later the wheat dies....probably lack of nutrients. What are your nitrate levels? Do you have an estimate for the dry weight of food you are using per day? Any guess on total weight of fishies?

(Numbers, numbers, I must have numbers.... Yesssssss! My precious numbers!)

One interesting number is 1lb (dry) feed makes about 30g ammonia (or nitrate). I'm sure it varies according to type of feed, etc, but is probably close for any fish. I'm not sure how this converts to plant biomass, but someone here must. Duckweed produces about 100g plant mass per gram of ammonia, but it is higher in water than land plants, so 50 grams might be a rough number. Let's see: you use about 60 or 70 grams of food/day->4 or 5 g ammonia/nitrate->200 or 250 g of plant growth/day? Then the plants hit the wall...

This is probably completely wrong: can anyone say "house of cards"?

Do read the RSG Filter thread here, hydrophilia:

http://www.backyardaquaponics.com/forum ... php?t=2173

Plants are not the only thing that can remove nitrate. Certain bacteria that can live in anoxic/anaerobic environments will cause it to offgas, too.

Yes, that's the principal behind a working septic system, but I didn't want to get into anaerobic systems, your posts on RSG filter notwithstanding. The RSG iron conversion concept is a nice variation.

Tony, does water siphon thru ur breeder system at all, when the pump is off, and return via a small sump pump?
My system has a ball valve return in the fish pump line. When this is slightly open, I get a clean siphon break at the end of the pump cycle. If this is closed I get a continual siphon effect and so, nitrate offgassing in the base of my gbs. My gbs do sit below my fishtank. (As an aside, I have never recorded any nitrate in my system to date, although I expect this to change as the fingerlings grow and the breeders are now pumping goodies into the system.)
I would also be interested in ur nitrite reading in ur breeder tank. If there is always some water at the base of the bio, u may have anaerobic zones and nitrate converting back to nitrite.

Thanks all for your replies and concerns.
The pump is too powerful for supplying just the GB, so I have a spray bar in the aquarium to take the majority of the water. This also acts to break any syphon, as the GB is lower than the aquarium.

The GB is a flood and drain configuration using a timer for pump operation. 15:45 on:off ratio (oops said that before). There is about 10mm of water which remains in the GB between fill cycles, but as the fill point is at the opposite end of the GB to the drain, I hoped that this would cause flushing of the retained water at the next fill.
Would 10mm of water cause anerobic areas in this configuration?

id say thats 100% fine tony. i worry more about CF through media (esp horizonal) systems where water finds the easiest path and leaves dead areas. F&D by natures solves that problem

Steve,
Thanks for the support.
Tony

My temp holding tank was using a 35 gallon drum/gravel/foam filter and I never had nitrates. I cant explain it. I think I will use a bio filter after my growbeds just to get the water good and clean.

Good idea DanD. I have not had a nitrate reading in my big system yet either! I am pretty sure it is bc of nitrate offgassing when the pump cycle stops but water keeps siphoning thru, and gassing off the nitrate! I have no scientific basis for saying this, only anecdotal observations.

Interesting TT, vb has mentioned something similar in his beliefs of nitrate off gassing...need some tests done to prove/disprove the theory...suppose there must be some anaerobic areas in the base of the GBs where there is constant water, especially in well established systems