Backyard Aquaponics
A place to discuss aquaponics

Hi all,

How do you determine an appropriate water cycling rate? ie. how often to pump water from the fishtank into the GBs?

Also, if you have 50 fish in 4000L, do you have to cycle twice as much as 50 fish in 2000L? (so the plants receive the same quantity of nitrates)

Scott.

IMHO, the higher your stocking density, the higher your flow needs to be. Rather than being for the plants, it would be mainly for the fish as more fish = more poo = more ammonia hence the need to run the water thru the biofilter more often.

Yeah, that makes sense Monya - thanks.

Now that I think about it, the flow rate doesn't matter so much for the plants, otherwise nitrates would equal zero.

So, given a low stocking density, how do I determine how often to pump water to the GBs?

Scott.

you're asking the tough questions scottie.......... for medium to high densities minimum of 2X tank volume per hour is accepted. as for minimum, i really don't know. i reckon we could reverse engineer a rate for you with the 6:2:1 ratio............just got back form work, if no one has done it a little later i'll try

With a small amount of fish it wouldn`t be too critical, the water volume gives a good buffer.
A 4000l tank would also have plenty of internal surface area for bacteria to take care of the bulk of the ammonia, (even without the growbeds) so the flow could be geared mainly for the plants.

Having a good tank turnover rate is good for putting oxygen into the water. Personally i`d go for an absolute minimum of one turnover every couple of hours just to keep the water moving if nothing else.

Scottie, it's not so much the quantity of fish but there approximate size/weight

Generally speaking, we work on maximum stocking density of 6kg/100 litres of fish water and having a growbed ratio of twice the volume of fish water.

So if we take the 4,000l tank, we can stock at a maximum of 240kg of fish and need 8,000l of media to keep the nutrients in check - for that you need an 8,000lph pump running 24/7, a lot of experience behind you and continuous management so as to not stress the fish.

Rather than keep waffling on (and go off topic ) - digest what is written and come back with more questions

Steve, what's the 6:2:1 ratio in reference to?



Ummmm ... I think I'll start with a low stocking density Monya! whadayareckon!?



Digested, all makes sense - thanks. Everyone here is so helpful, muchly appreciated. I might not be posting much but I spend hours reading the posts in this forum - so much information & inspiration! Awesome!

So, next question ... if I'm going to have a ~4000L tank (as seems likely at this point) & I need to do a water change ... well .... that's a lot of water Is this manageable using the mains water supply? Can the chorine & chloramine (?) be neutralised quickly? I don't have a rainwater tank yet (& even if I did it would probably only be ~3000L which ain't gonna allow me many water changes).

Scott.

Scottie - if you are sensible and cycle your system with a smallish number of fish, even even better cycle without fish (I would recommend urine), then you should never have to ever do a water change. Only water top ups will be required to make up for water lost through evaporation and transpiration.

I buy my neutraliser from Woolies in 500ml bottles. Each bottle will do 2000 litres and they don't cost much.

hehehe - 240kg fish is a lot of fish so best to scale down, I'm working on 100 fish growing out to ~500g in ~2,400l of water and ~3,000l of growbeds.

That will supply fish for a year for 2 people and veggies for the whole neighbourhood.

Water changes - I haven't done a water change in 6 months, once the system has cycled properly it is a matter of balancing the system and doing water top ups - 4,000l tank would be about 110-140litre of water/week (plants use most of that)

Chlorine/chloramine can be neutralised straight away, aquarium shops sell bottles of chlorine remover at a reasonable price

6:2:1 ratio - I think Steve was referring to
6kg of fish/100l
2 cubic metres of growbed to 1,000l of water

....I agree with wot vb said

Alternatively, you could say a maximum of 9kg of fish per square meter growbed area, because the growbed area is the limiting factor in most systems.

You need to pump around at least 100L per hour per 3kg of fish.

(Based on 6:2:1, which I think is based on a 30cm high growbed?)

i think you have done your maths wrong at some point.
The ratio is max. 6kg of fish per 100l of water with 200l of growbeds attached thus making it 3kg of fish per 100l of growbed.

I probably started the stuff up with that ratio!

it should have been 6:100:200 or 60:1000:2000

Fish Kg : tank liters : grow bed liters (max figures, half fish kg for safety margin)

And with a 30cm high growbed that means 9kg of fish per square meter of growbed.

yeah sorry my mind thinks in the most simple terms,