Backyard Aquaponics
A place to discuss aquaponics

I would like to know how many cubic meters of gravel per cubic meters of fish water assuming a stocking density of 100 fish per cubic meter and a growbed of 2:1 (2 times more water in growbeds than fish water). I guess the gravel filter will be after the growbeds, but there will be a small filtration system prior to the growbeds.
What would you guys recomment 2:1? 3:1. Im looking towards a commercial facility. Thank you guys.

100 fish in 1000L is a LOT of fish. But what matters is how much you feed them and the composition of the feed. Also what kind of fish?

The 1 fish per 10L is a figure quoted by a lot of start-up sites, but you onl;y have to look around these forums to see the number of new-starters who lose their fish. The smaller your system the faster things can go belly-up (I LIKE puns, OK? ) so if you are going to try for that figure you better make sure you have capacity to spare.

You should check with the more experienced folk around here but I don't think 2:1 will cut it. I'm not sure as to your reasoning for putting the gravel bed after the GB's - it seems to me you'd be removing almost everything the gravel bed is designed to handle.

My inclination would be, ramp up the filtering before the GB's (have a good sized solids filter AND a decent biofilter) and turn your gravel bed into another GB.

Also add in lots of aeration, using air pumps and venturis, and have failover systems for each power usage.

Then cycle the system fully without fish and then add maybe 20 at a time and watch the water quality.

Or, start small, maybe the FT and GB with 20 fish and see how you go - you can always expand once you get things working.

I think there was a small confusion. By gravel bed, i meant gravel filter after the vegetable rafts. I was thinking of a raft system, where the water flows from the fish tanks through a small filtration system, then to the raft system (veggies) then through a gravel filter and then back to the fish tanks (but i dnt knw gravel ratio (gravel is extremely cheap in my country)) Im thinking of tilapia.
When you say "good sized solids filter AND a decent biofilter" what type exactly do you mean?
Thank you.

For solids you need to know how much water you're putting through. A settling pond needs water moving slowly for quite some time to allow the solids to settle out. A swirl filter requires MUCH less time because the vortex spins the solids out of the flow.

So if you are pumping at (say) 1000L per hour, you might want a retention time of 2 minutes in a swirl filter so if you have (say) a 100L swirl filter you divide the rate by the barrel size - 1000/100 = 10 lots of 100L per hour moving through your filter. That's a 6 minute retention time. (60mins/10 changes) Clearly you can make the barrel smaller for a lower retention time. (e.g. 40L gives 1000/40 = 25, 60/25 = 2.4 minutes)

Or you could raise your flow rate to reduce the retention time.

That just gives you an idea - I think (don't have evidence yet) that having a decent retention time is fine, provided the water is moving rapidly enough in the barrel for the vortext to work.

There are other types of solids filters - have a look at DIY Swirl Filter and browse the site for other ideas.

For a biofilter I am making a Moving Bed Biofilter - it uses bioballs to filter biologicals after the solids have been removed and before the GB's.

I suggest even with rafts you want your filtering done before the water gets to the rafts. Less clogging for one and cleaner water means less other growths to compete for nutrients.

All of the above filter suggestions are perfectly valid....

But all filters require cleaning... and of all the filter types suggested... or in existance... a gravel filter would be the worst possible example.. from a maintenance point of view....

And I'd bet my house... that you wont find a commercial aquaculture operation running gravel filters... at any size/density...

All solids processing... and filters... have an oxygen overhead...

And here's the main reason for NOT stocking @100m3..... the oxygen requirement for the fish themelves (even allowing for tilapia)... the conversion of feed to protein... plus the bilogical oxygen demand of waste conversions....

Means you just can't provide enough oxygen at that level of stocking... or even half that.... by normal air aspiration...

You would need a direct oxygen injection system.... and a damn good one... either bottled/tank supplied.... or automated mains power supplied, with backup generators...

Now, consider the capital investment of a direct oxygen injection grow out facility @100m3, drum filters, feed and electricity costs etc...

To grow Tilapia.. with a low market per kilo return.... why do it???

If your desire/answer... is that it's not about the fish being profitable...(but lets assume it is)... but that it's more about the profit from the plants... then why not just duplicate the exisitng low density hobby farm approach....

You don't need a truckful of fish... to grow a lot of plants...

A properly designed 5 tonne a year RAS operation (probably not viable in it's own right).. @ 20-25m3 standing biomass...for example.... could easily provide enough nutrient provision for 150sq mtr of plant production... over 3000 plants (at 20 plants per m2)

Edited : In fact... there's enough nutrient provision for about 3 times that number of plants... but you'd need 3 times the growing area... :wink

I'm always incredulous that we happily accept the definition of aquaponics as a conjunction of aquaculuture an hydroponics...

And then basically refuse to design a "commercial" aquaponics system... by totally ignoring the proven designs.. and academic research.... of both the sub components...

If anyone wants to grow a lot of fish... design your system on proven RAS principles and designs....

Ignore the backyard aquaponics self appointed "gurus"... and training experts... that think that somehow they have reinvented the wheel... and can magically invent a better mouse trap... when applied to a commercial scale... it's nonsense...

What's your definition of commercial?
How big are we talking here?

I thought you rented ?

Rupe's definition of commercial...

...imagine a space station, then you are getting close

viewtopic.php?f=4&t=15304

Nice to know where we all stand in your estimation Rupert.

Damn shame all that knowledge garnered by so many people over so many systems counts for so little in your view.

Could you perhaps tell us where we should all go to find the true gurus we need to tell us how to do things according to the Gospel?

I think Rupe meant: to be cautious of those who profess to be gurus. Rupe is just a proponent of empirical data.

OK... I'll take your word for it MacGyver - it doesn't read that way though with the 'backyard' thrown in there...

My comment wasn't being directed to fourm members Journeyman...


Nor in any way related to the wealth of information provided by backyard aquaponicists worldwide....

There's a vast difference between what we can acheive in our backyards... and how we do it....

Compared to what is required to do "commercial aquaponics"... and how to do it.... and the scale involved...

And the concept that you can just "scale up" a backyard system... or methodology... just isn't valid....

And I've voiced that opinion... and questioned the scale, and model of the so called "commercialisation" of aquaponics.... consistantly for several years....

And I do so from previous experience in a hydroponics operation... and some knowledge of RAS operations... and what is, and isn't viable... and profitable.. in a commercial sense...

And it's not hard to work back the numbers... of how much fish, and numbers of plants... can be produced annually...

How much both/either are worth.. and approximate costs.....

And the numbers just add up.... and I'm not the only one who says so....

Even if I do so more often than others...