Backyard Aquaponics
A place to discuss aquaponics

Interesting questions. I'm turning over the tank volume 3X per hour. I have 1:1 tank:growbeds, and don't have space to increase the growbeds. I still have problems with trace amounts of ammonia and nitrite, although I only have about 30% of the fishload I think I should be able to handle. I'm working on getting the nitrates down by increasing plant load, decreasing fish count, and adding some sand columns to increase nitrogen offgassing. My hope is that as things get into better balance, that the ammonia/nitrite will settle down. I'll let everyone know how that goes eventually...in the Conversations with a Really Smart Guy thread.

Scottie I was pumping the water more initially and found this helped to cycle the system and keep the water clear, but later I was able to wind the timer back quite alot without sacrificing the water quality. I could probably wind it back even more if I put an air pump in the fish tank but at the moment having the water circulate is the only way I'm picking up oxygen.

Please pardon my stupid question, but should'nt you be referring to GB capacity by cubic rather than square meters? (As the depth of the GB is as important as the surface area in determining the working volume.) Just my ignorance showing.

Kevin

Kevin, both volume and surface area are important for different reasons. The volume gives a better indication of how much media there is to act as the biofilter. Depth is also important to make sure the bacteria can stay within the required parameters (ie they don't get too hot or dry out) and to give plants support. Grow-bed surface area becomes important because it is what determines how many plants you can have to draw out all the nitrates.

This is not an exact science and the 2:1 ratio and 6kg to 100 litres is dependent upon so many other factors such as those above as well as planting density, types of plants, feeding rate etc.

Thanks VB, that is exactly what I was thinking. (If I had a GB or tank that was the proper number of sq, but I was only filling it a couple of cm, then I would not have a proper depth to provide the carrying capacity required by either the plants or of the fish.)

Personally, I think it might be better to refer to either GB or fish stocking densities by cubic rather than by square surface area. Along with this should go a minimumal depth recommendation, to provide a standard from which it can be increased or decreased to suit the individual application.

Kevin

I thought that the optimum growbed depth was already established at about 30cms. Anything deeper would be a waste of space (plant-wise, anyways), and anything shallower would get too shallow for the roots.

For fish it's the volume that counts most, for bacteria as well (or better still, the total surface area of the growbed / filter media), but for plants it's the surface area of the growbed that counts most, as their ability to remove nitrates is limited by the amount of sunlight they get. And because in most systems the ability to remove nitrates is the limiting factor, I thought it'd be easiest to think in surface area for the plants, instead of volume.

Also, by using square meters instead of cubic ones, you can easily add an area with for instance lettuce floats on top of a tank to the calculation (but then you might not have enough media for the bacteria.. Ack!)

Optimum is about 30cm for growbeds, but you work with the containers you can get.

That is what I was thinking, that there is an optimal depth, which can be the standard from which you can work. But when you are describing the capacity of the GB in relation to the fish tank, then it is better to refer to it in cubic measurement.

As we are aware the generally accepted minimumal ratio is 2:1. (GB's are twice the area of the fish tank; i.e. if you have a 3000l fish tank you will require a capacity of 6000l in GB.) Now, if my memory serves me correctly one liter is one thousand milli liters and one ml is equivalent to one cubic centimeter. So if you have a 100l fish tank you will need 200l of GB, which is equal to 200,000 cc's, so by dividing the cc's by 30, and then by finding square root of the sum you can calculate the desired amount of surface area. Hopefully someone can advise me if my method of calculation is off. I know that this is old hat to many of you.

Thanks,
Kevin

Hi Kevin,
If you want a square growbed, that would be right.

Consider the maximum width that you want to reach into the bed for harvest. Rather than taking the square root, divide by one meter if you can walk on both sides of your growbed (maybe 50-60cm if accessible from only one side). Then you can construct a bed one meter x the resulting length x 30cm deep.

Hi Kevin,
nothing wrong with your maths, the general standard we use (I do) is 1,000 litres is equivalent to 1 cubic metre, this fits more readily to the figures of tank capacities.

There was a discussion on another thread as to when grow bed or fish tank capacity should be used with the fish weight, basically:
1) calculate 6kg for every 100l of water
2) calculate 3kg for every 100l of grow bed (2:1 ratio)

use the lowest figure.

Those with larger fish tanks find that the ratio is 1:1 or less, I have (when fully set up) 2,400l of water and 2,800l of GB, so will be working just over 1:1....have no idea what Jaymie's is

30cm depth is what I use but there is always exceptions....if you were going to grow lettuces only then the depth could be decreased to possibly 10cm...I suppose this would be for a commercial type endeavour tho

Good points raised all the same

Thanks Janet and Les for confirming my calculation method. The reason I went the sq root method, was the most expedient means to an end. Obvisouly if the square GB is not suitable, due to width, then it would have to be adapted by length with the limiting factor of desired width. (Simply subtract the desired width from the answer and add the result back in order to get required length.)

Kevin

Now I think your math is off, Kevin. (Or at least expressed oddly.) If you calculate that you need 25 square feet of bed space, and take the square root, you would have a 5' x 5' bed. Since 5' is too wide, you decide you want a 3' bed. You can't just move the 2' and make a 3' x 7' bed, since this is only 21 square feet. Instead, go back to the number that you took the square root of (in this case, 25), and instead, just divide it by the width of the bed desired. So if I want a 3' wide bed, the length must be 25/3 = 8'4".

I've got about 10,000L fish tank, 9,000L grow beds. I haven't had a chance to time the system with the new sump pump, but I think it cycles through the tides every two hours, 12 times a day.
I have 250 fish.
So, 125Kg fish : 9000L GB : 10,000L FT

0.125 : 1 : 1

but because the system cycles at a lower rate, my stocking levels are lower.

Now I'm just getting lost with all the figures....

Don't get too hung up on the ratios of growbed to fish tank, I've found it's better to take into acount the number of fish per growbed volume, or more to the point weight of fish per growbed volume..

In the systems that I've been building lately with my 500L growbeds, I've been working on 25 fish per growbed for a recommended stocking rate.. This is a nice safe sort of stocking rate that will give some good growth rates of plants but not over stress the system at all.. This is with a cycling rate of about once an hour..

Would once an hour be the growbed volume, or the fish tank volume?