Backyard Aquaponics
A place to discuss aquaponics

I realise for the most part people work this out the other way around and design system to be over what is required for amount for the fish (food) in the system. But how much is too much. At some point the amount nutrients in the cycled water must become so low as to be of no use to the plants what so ever (?).

I was given a rule of thumb that a media bed can process 30g of food per m2 of media (assuming 300mm/12" depth). But in a micro system where you are providing only 100 - 500 mg of food/day how much media is too much?

Essentially how much growing could a few goldfish in a 40L (10.5 US gallons) tank give you.

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Sounds like you are trying to over plan...
Remember that you can supplement the fish poo with organic supplements .. eg. SeaSol etc. Etc.
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To give you an idea, the system in this link had 10 fish powering it.

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