Backyard Aquaponics
A place to discuss aquaponics

I still don't like the 2:1 recommendations myself much, at least not for the newbie types, I prefer to see larger volumes of water, my favourite it 1:2...

I know, I'm a troublemaker.

But larger fish tank volumes means greater system stability and so better chance of success for newbies.

The water velocities are higher which allows the flow to more effectively transport the solids.


Generally speaking it is unnecessary especially if you have adequate media beds to handle the solid wastes. If you were to say have rather narrow GBs and only add the solids at one end it can be an issue and a distribution grid could alleviate the issue.

Personally in my 3:1 ratio system stocked to 25kg/m3 with very narrow aspect ratio bes (1:16 ie 1m wide 16m long FT effluent added at one narrow end) it has not been a big enough problem for me to bother to do anything about it.

+1 to what the guys have just said.

Oh so often new people are given rules of thumb and then want to push the limits and or skimp on their design. As soon as you do either or worse both you are heading for serious trouble.

The 2:1 rule is a seriously over simplified design guideline which is almost never stated with the associated caveats that go with it. Design a system using the 2:1 rule and you will have a system design that is capable of being stable and reliable system. For BYAP systems reliabilty and stablity are the two primary features that the design requires. BYAPers generally speaking don't have the money to spend on monitoring systems that allow for higher productivity and less redundancy built into the design. Similarly they don't have the time to be fiddling with the system and they will sooner or later want to go away for the weekend or go to work during the day. A system that isn't stable can't be left alone for days at a time without coming back to a lot of dead fish sooner or later.

As soon as you stray outside the bounds of the 2:1 rule it no longer applies and since we don't really know where the boundary is newbies are advised to move away from the area which we know is dangerous.

May be we should restate the 2:1 rule as a minimum GB:FT ratio?

Doing so is likely to bring a whole stack of 1:1ers out of the wood work though.

NO.....!!!!!!!!!!


Bring on the 1:2 rule....

Yeah but then you have to explain that they can only stock the FT as if it was a smaller tank equal to half the size of the media volume they have

Then you have all those other rules of thumb to deal with on number of fish per 1000L of FT.

I agree it does make for a stable system and a small number of fish in a large tank can solve problems but in practice how many systems have we seen like that that are ridiculously overstocked? Then their fish die and then they say that GBs don't work and the you have to include solids removal components.

But people should NEVER be stocking their fish to the size of their fish tank anyway....

Stocking to a fish tank size is WRONG, always...

Yes but how often do people do it anyway.

Thats why were doing what we do....

To counteract those who have given crap inforamtion...

lol. TC already answered me but I imagine you're alluding to an indexed system such that the pumps flow isn't equally divided amongst all GBs? Though I think other factors might limit how fast you can fill the GB.



The context was long narrow beds so yeah that was the conclusion



The beds TC was talking about were less than 2 feet wide iirc. So it's a bit different than your setup. As yours is still nearly as wide as an IBC bed.

So what you're really saying is 1:1 but with stocking as though it were 2:1 Though I can honestly see how stating it that way would cause confusion. People see a ratio and stop reading.

*edit* err nvm. That's not what you're saying at all. More like 1:2 with stocking as though it were 1:1?

You know I've been wondering for a while why the ratio is stated in the form GB:FT?

Since we're using fish effluent to feed plants it seems to me it would be more natural to say it as FT:GB. /shrug. Terminology and nomenclature are sometimes strange in how they evolve over time. Keeping to a standard is more important I suppose.

No I was talking about timed flood and drain systems.



The beds TC was talking about have an aspect ratio of 1:21. Functionally there is not much difference between hers and mine. While my beds are 1m wide they are 16m long. Proportionally her rate of flow is higher because the friction losses from the gravel are such that the friction loss through the gravel is greater than the fall but this would mean that her beds would be less likely to have localised clogging because when the flow goes over the top of the gravel solids would be distributed along the length of the bed.

I don't have any problem with the idea EB has of having a big fish tank and a reasonable size grow bed with like the 1:2 grow bed to fish tank ratio, and only stocking x amount of fish per grow bed etc. AS LONG AS the information about how many fish to stock goes right along with it and since BYAP does that their systems work well. (Wish we had a BYAP store over here to tell the truth since we probably would have purchased lots of stuff from them over the years.)
When I started this thread way back when, it was largely to put together in one place, all the things we were all repeating constantly.

More recently, there are new things I have been learning with my big systems.
Having the greater amount of media beds, especially flood and drain, and less water does mean there are more fluctuations in the system. I'm not even talking about the chemistry so much as temperature. Especially in climates where the day to night fluctuation has a high differential. During certain seasons in my location (spring time when it is still relatively dry and it can get darn hot during the day but still cool off at night) the temperature swing in the fish tank is about as wide in that 3:1 system as I think the fish are willing to tolerate and still eat well.

When people are designing a first small system, it is rare that they are going to manage a 2:1 system with a fish tank big enough for temperature stability outdoors here.

I generally always recommend to people planning an outdoor, not temperature controlled system in my climate, that they should go for a minimum of 300 gallons of fish tank. And then only stock as many fish (that grow out to 1 lb) as you have cubic feet of media bed. So in my quarantine system (that is right up front as people visit the farm and it a handy example of something more "backyard" size) which has 300 gallons of fish tank and 32 cubic feet of gravel, I would recommend only stocking 15 channel catfish because I figure most newbies are not going to grow them past 2 lb in their first season.

I think all the discussion is missing the point of as well as (1) ensuring adequate solids mineralisation capacity in the GB AND (2) biofiltration capacity (generally this one met if the mineralisation requirement met), there must be (3) enough growing area to have enough plants to deal with the amounts of N being produced by the feed rate.

If this requirement is not met, then the plants won't be providing enough alkalisation to counteract ph fall and the nitrates will increase overtime and push the growing system out of balance.

There doesn't seem to be any consensus on what the adequate solids mineralisation capacity is... other than saying that the 2:1 ratio is enough for most use cases. Try to pin down what most use cases means and you'll get different answers. Yeah, biofiltration is usually a non-issue in a MB system as the media itself typically has plenty of surface area for bacterial growth. Though there's some varying views on just how much of the media volume is usable in a ebb and flow it's still accepted that with 30cm deep GB's that you'll not run into issues. Just how much biofiltration MB's can provide other GB types in a hybrid system is something of an enigma because there doesn't seem to be an accepted model for either Solid/Bio filtering in MB's. Your last point is one that I wish rules of thumb more directly dealt with but the variables involved keeps it from being a simple answer i.e. Feed conversion rate of the fish, type of feed, temperature, plant types, plant density, fish species growth rate, mineralisation/supplements, and fish/plant seeding/harvesting style are but a few of factors to consider. It seems like most people wing it and slowing populate the system until they can find an equilibrium.



afaik plants do not provide any pH increase. Usually it's either topping off with source water that has a high KH or slight adjustments using bases that keep pH levels balanced. Nitrates typically increase due to a insufficient amount of plants compared to fish. Though I imagine other factors can contribute to nutrient uptake (micronutrients, etc).



Temp swings are one of my worries during the winters here. We typically don't get below freezing very often but daytime temps of 50-70 and nighttime temps of 30-50 are cause for concern. That's why I'm considering some passive solar heating during the day, larger FT, and possibly a different fish species than the norm. I can't seem to find my notes of fish but off the top of my head carp/koi and largemouth bass seemed to have a wide temp tolerance. I know there are other considerations. I had only begun to look at the options. I know bass are problematic being carnivorous. Higher protein feed and a need to isolate by size. Meaning multiple FT's to be practical. Also read carnivorous fish tend to have bad FCR compared to herbivores/omnivores. There were some water quality quirks too I know.

That is becuase no one has really studied it which means that we have to rely on the anecdotal evidence available to us.


Actually this is something we do know. Media beds were used as filters in the early days of recirculating aquaculture and if you just want a media bed to be a bio filter there are good models to use to calculate your requirements.

Timmons et al or Le Kang would be a good start.

Having said that the aquaculture engineering formuli in these texts are themselves rules of thumb. They are more detailed rules of thumb than the 2:1 ratio but like all engineering guidelines they are fairly conservative which means that if you use them your biofilter will be over sized. Just the way engineers like it.


Even in RAS while the engineering is complicated it is still just a rule of thumb with pretty heafty safety margins included in the calculations. For a BYAP system going to that degree of calculation is a leisure exercise.