Backyard Aquaponics
A place to discuss aquaponics

We are helping Tom's sister & brother-in-law setup an aquaponics system. This will be using an existing hot tub that is in a sunroom. The hot tub is 450 gallons (1700 liters) but there is only space for about 35 square feet (3.25 sq. meters) of grow beds. As I recall, the ratios run 1 fish per 2 gallons of water per 4 cubic feet of grow bed. That would be 9 fish here. The question is whether you can get more fish in a system of this nature because of the volume of water involved. And, if so, any ideas on an estimate of what would grow well? We do have lots of fish to "play" with from the order we got this summer, but I don't want to be a fish killer. :p

Stocking density is ultimately determined by your filtration capacity... not your tank water volume...

Yep ... you could stock the tank with how ever many fish you wanted... if they're really small...

But feed them... grow them... and they'll poop..... it's about filtration capacity....

Removing solids... ala Frank's recent discussions.... will help....

If number of fish, is all that counts... then external bio-filtration, solids removal... ala aquaculture... can be done...

If it's plants you want as well... then you'll probably need to retain the solids for trace element provision, and have xxxx amount of growbed volume to provide your bio-filtration...

I thought the ratios were like 2:1 grow bed volume to fish tank volume
The numbers I seem to remember were 2-3 kg of fish per 100 l of water per 200 l of grow bed.
now I'm not so good about how that would shake down into pounds and gallons.

I do know that a cubic foot of gravel is closer to 7 gallons.

So if you wanted ......

Hang on need to look some stuff up

Okay. Thanks TC. So, should be more like
1 pound fish per 5 gallons water per 10 gallons growbed as a ballpark estiimate.

That makes more sense and would be around 26 fish in the system.

That's what I get for using my memory instead of looking the stuff up.

If I've worked it out right.. assuming 300mm deep growbeds over 3.25 sq mtr... that's about 1000 ltr of growbed...

So less than a 1:1 ratio.... 3kg/100ltr x 1700ltr = > 51kg... or 102 x 500gm fish....

Because you're limited by your growbed capacity of less than 1:1 (about 75%)... then that's about 75 fish max.... by my reckoning....

I wouldn't go above 50 x 500gm fish (1 pound) fish.... max.... for a stable system....

ok so 35 Square feet of space for grow beds, if the grow beds were only a foot deep, that gives you like 260 ish gallons of grow bed capacity. If you can do deeper grow beds, like 2 foot deep, you effectively double your filter capacity providing we are talking about flood and drain.
so with 520 gallons of grow bed to that hot tub, your are now close to the range of the 1:1 capacity that is pretty common. If you don't have a sump tank in the plan, you can't really do flood and drain with much better a ratio without leaving the fish in shallow water. How much fish, well I don't know the numbers for gallons or in pounds so let me convert.

I'm going to just use half the grow bed volume (260 gallons) to estimate the fishtank that would provide high density filtration for. 984 l Well if you can have 2-3 kg of fish in 100 l in a good AP system, then 9-10 times that (providing the keepers are good fish keepers) that should be that system could have 18-30 kg of fish. That would be 39-66 lb of fish.

Please some one correct me if I'm wrong cause these numbers seem pretty big to me and makes me think I could be wrong here.

So even if you just went with standard 1 foot depth of 35 square feet of bed doing flood and drain, it seems you could still have between 9 and 15 kg or between 19 and 33 lb of fish grow out in that tank.

If you want to push much higher stocking densities you start needing to talk about straight O2 injection, external bio-filters, consistent regular water changing, etc.

Now you might be able to add some plant pots filled with gravel around the benches and they could flood and drain as the level in the tank fluctuates giving you some added bio filter within the fish tank. You just want to make sure the fish don't get trapped high and dry up on the bench if the water level drops that low.

Yep... you're both right... for a small, "new" system about 25 fish is probably a safe bet...

So, one last question... (maybe)

How does a floating raft affect the filtration?
How do you calculate for NFT?

Okay... so that's two questions. Ah well.

Floating raft systems, usually remove solids prior to pumping the remaining nutrient water through the raft growbeds.... how much filtration does the raft component provide...

Some, but I don't know if/how it's been calculated... the system style utilises the water bourne suspended nutrients....

The raft surface would no doubt provide some nitrification surface area, as would any media that the plants were in, if any.... ie... net pot with hydroton inserted through raft...

Safe to say, probably negligible, unless the rafts are really long > surface area...

For the limited space available.... nowhere near the same as growbeds... unless you plan to remove solids, etc...

NFT.... again doesn't really provide filtration as such... essentially the same as rafts....

Do however need to factor in the volume of any NFT trays in relation to water out of fish tank or sump....

If you make the GBs deeper you get more bio-filter per m2 of gb. This ratio of plant growing area to fish can then be corrected by floating rafts and/or NFT.

For example:

1000L FT
2000L Gravel in a grow bed .90cm deep by 2m by 1.1m.
GB growing area equals 2.2m2 instead of the 6.6m2 that you would get with 30cm deep beds
NFT and Floating raft growing area of 4.4m2 (6.6-2.2).

I have explained that well enough

Crap



Where's the #&$% button

I meant

You can have as many fish as you like in a small amount of water, but if something goes wrong, its wrong really bad. More water, less fish, more stable.

10 x 1/2kg fish per 100 litres of growbed, once the system has matured. The amount of water that the fish get is up to you. Stocking 5 fish per 100 litres of growbeds = a near zero maintenance system, 10 would mean more pump and tank cleaning etc.

If you do the double depth grow beds and stock fairly high for that amount of gravel, then you may find that a raft over the fish tank or a could nft troughs snuck in some where will help pull out more nitrates since the surface space for planting in the 2' deep grow bed is kinda small for it's bio-filter capacity. Then again, house plants and dirt garden plants like a drink of system water regularly too and this can give some of the regular water change to keep nitrate under control too.

Does make sense... thanks! It's why I love this group so much.

thanks Rupert
here is a link to the posts:
viewtopic.php?f=14&t=501&st=0&sk=t&sd=a&hilit=worm&start=15
the discussion starts about halfway the page
(I thought it good to post this link because this forum is HUGE and I will never find the time to read it all, though I should. I would ask you all to do so, saves heaps of time)

Increasing the growbed depth will increase your biofilter capacity but will not remove the nitrates from the system which will build up over time and (while not toxic to fish) might cause stress to the fish

you can only solve this by water exchange, thus by letting go contaminated water to drain
we don't want that, or do we?

removing part of the nutrients (i.e. to feed to worms or to a -possibly outside- duckweed pond) is a better solution

to my opinion

Frank