Aquaponics - Split system

A rule of thumb ratio to stick to is 2 grow bed : 1 fish tank. This allows a big enough population of nitrifying bacteria to process the maximum safe population of fish.

The maximum safe limit of fish population is 3 kg for every 100 litres of water. since a litre of water weighs 1kg, we can say that 3 % of the fish tank's weight can be made up of fish. That's a lot of fish for a cubic meter of space.

The issue with the 2:1 ratio of grow bed to fish tank, is that the grow bed takes a very large percentage of the fish's water. I'd like to grow some marron as well as all these fish, so I'll want even more grow bed. Even more grow bed means my fish will be spending some of their day walking around on dry land.

Fish hate dry land.

One possible solution to this is to add some extra grow beds down at the sump level. My aquaponics space is on a flattened area of sloping ground, so its easy for me to add grow beds down the hill a bit at the sump level. This means the grow beds can act as extra sump capacity rather than being a drain on sump capacity.

When I add my marron farm, I intend to add some blue barrels as large planter pots for a few fruit trees. I'd love to grow a lime, and some blueberries, and perhaps raspberries or grapes.

My description is lacking, but luckily I spent all morning making an animation to demonstrate what I mean.

• The pump is on a timer and runs for 10-15 minutes per hour.
• Water is pumped from the sump up to the fish tank.
• We make the water exit the fish tank from the centre at the bottom because with the pump creating a bit of a whirl pool, that's where all the solid fish waste will gather.
• The water and solids from the fish tank enter the swirl filter. 
• The swirl filter has an inlet port half way up and and outlet at the top so the solids sink to the bottom and are left behind. In the event of some kind of "over nutrient emergency", I'll be able to remove the days solids to lessen the load on the system. If all is going well, the solids in the swirls filter will be chewed up by a few cups of shell grit until they are small enough, and light enough to float out into the growbed. This should help distribute the solids, allowing them to easily flow to all areas of the growbed.
• As soon as the growbed starts to fill, it also starts to empty via hole in the base of the standpipe, but the inflow rate is higher than the outflow rate so the growbed's water level rises.
• Once filled, the open top of the standpipe allows water to flow free back to the lower level to prevent overflow.
• The pump runs for a while longer. The length of time the pump runs for is determined be the need to turnover approximately one fishtank of water each hour. This helps oxygenate the water, and allows the bacteria to get a look at the ammonia, and nitrites.
• The pump shuts off.
• The grow bed continues to drain over the next 40 minutes or so.
• When the grow bed is full, the planters are drained. When the planters are full, the growbed is drained. This way we introduce extra growbeds into the system without needing a larger sump.
• Water flows freely between the planters, and the sump. 
• When evaporation or plant use drops the water below the trigger level of the float valve, water is added from my concrete rainwater tank. My concrete tank water has no chlorine so it can be added directly. It also has a similar pH to my aquaponics system. If you use mains water, you need to have your water sit for a couple of days in the sun to turn the chlorine into something less harmful to fish. [see comments]
• The cycle repeats.