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[Toby]

Hi, newbie here toying with the idea of an AP setup and how I could fit it in the yard. Before I get too far I'd like to hear people's thoughts on a few things.

For a system with a 1-2000L fish tank am I better off with a:


continuous, or
flood and drain

system. Personally I like the idea of 2, plus you get to build cool things like auto-siphons :) . To me it seems filling the grow beds with FT water and letting it sit a bit will allow the plants/bacteria a greater chance to extract nutrients vs. 1.

Now if I chose 2, is it better to flood the grow tanks:


fast, then drain slowly or
slow, then drain quickly

Not sure on this one, but (b) probably fits my early ideas of a system better.

Now, independent of the choices above, is it better to drain water from the fish tank:


fast, or
slow

My thoughts were to go with 1, because I thought if I had e.g. a siphon draining from the floor of the FT it'd do a better job at removing solids if it was sucking at a fast rate vs a slow draining system. Does that sound right?

Thanks, I might have more questions based on your answers.

[Murray]

Hi Toby,

It is possible to have continuous pumping from the fish tank with flood and drain by using auto siphons. In practice it does not matter much if you flood fast and drain slow or the opposite.
I have systems that run on timers and flood very rapidly, moving 750 litres of water into grow beds in 5.5 minutes, then drains back via a sump in 50 minutes.
I also have systems that run continuously to flood the grow bed, and the drain is done rapidly by the use of an auto siphon. That complete cycle is about 12 minutes.

The essential element of flood and drain is to do just that, flood and drain. Flooding delivers the nutrient laden water to the grow bed, (fish solids and ammonia for conversion), and the drain cycle allows oxygen to get into every corner of the grow bed.
All this equals healthy plants and fish .
Pick the method that suits you best.

We are moving more away from big pumps in order to conserve power, so that will lead more towards continuous flow using auto siphons.

[Toby]

Thanks Murray. OK, I do have more questions I forgot to ask but first I'll describe the vague ideas I've got.


1-2000L fish tank

overflow tank e.g. 1/4 the volume of the FT

grow beds in a 2:1 ratio to the FT

sump (same volume as overflow tank?)


And these will be linked in the following way:


FT will have a siphon off the bottom into the overflow tank

overflow tank will have ball valves to GBs to control the flow rate of the system

(ii) implies slow fill, fast drain of GBs

GBs will either be all linked to equalise water level w/ a bell siphon in one GB or all GBs separately controlled by bell siphons (siphoning into the sump)

sump will have a float switch controlled pump that evacuates the whole tank when it fills, which'll pump back into FT, which'll activate (i), which'll help clean the bottom of the FT


The overflow tank can double as a fingerling tank, provided the outflow tap is high enough in the tank that some water always remains in there. Might be an issue with buildup of the stuff sucked off the bottom of the FT but the smaller tank will be easier to clean. I haven't thought about a biofilter yet.

Using a ball valve off the overflow tank to regulate flow means that at the sump end if the ball valve flow was high the pump would operate a lot of the time. If the flow was low the pump would operate infrequently, so you could tailor the system's flow to suit the pump. E.g. a big powerful pump would evacuate the sump quickly and fill the overflow tank quickly. A little pump would struggle to drain the sump and struggle to keep up with the flow from the overflow tank so it'd operate in a more continuous fashion.

A further implication of this design is that I make sure that the bell siphon(s) drain less from the GBs than the volume of the sump. So here are those questions I promised earlier - how much should I ideally drain from the GBs each time in a slow fill/quick drain flood-and-drain system? Is it OK to only drain e.g. 1/10 (200L from 2000L of GBs)? Or should it be closer to 9/10 (in which case this whole design isn't feasible any more)?

[Murray]

Toby, I did a very detailed post to you a while ago, I went away to talk on the phone and when I got back it was gone...sorry.

But in brief, stick with flood and completely drain. Use a method that has a long track record of success before trying some new idea. There are countless variations to that theme that can be experimented with, but is good practice to start with a known and proven method.
It's best not to try and re-invent the wheel...not at first anyway.

Using auto siphon is great and you will find that there will always be 25 to 30 mm of water left in the bottom of the grow bed anyway when the auto siphon cuts off. If you leave the grow bed 1/3 or 1/2 full then you will parts of the grow bed that will become stagnant, and plants will not do as well. Plants will still grow, but not as well as they could have

2 to 1 ratio is OK for an established system, but 1 to 1 is more realistic for a new system.
In our balcony kits the ratio is 1/4 to 1, that is 1/4 grow bed to 1 fish tank and they work just fine so long as you don't over stock or over feed.

It is possible to go way past 2 to 1. In commercial systems that is pushed way beyond that. For example a system we designed and built for farm in Mildura has 8 thousand ltr of fish tanks and close to 28 thousand ltrs of grow beds/troughs. That is beyond 3 to 1.