Aquaponics 101 – Soaking up the Nutes

My ongoing battle to reduce my Aquaponic systems Ammonia and Nitrite levels can be resolved in three possible ways;

  • Reduce the number of fish.
  • Dilute the water. ie. do regular water changes.
  • Add more grow beds.

The problem that I have, is that my fish are producing more nutrients than my vegetables can soak up. These “nutes” will cause medium and long term problems in the fish, and have be addressed as a matter of urgency. The rule of thumb is 2 – 3 times the volume of gravel growbeds to water in ponds. Even if I ignore the sump, my two ponds still hold 5800L of water. My growbeds currently hold 12 x 105kg of gravel ie. 1260kg, so as you can see, I am way off of the mark. I should theoretically have over 11 tons of gravel !!!

This is good news, because as you have probably gathered, building growbeds is one of my favourite hobbies.

We started by cutting a barrel-width shelf behind the greenhouse.


Then I plumbed up six more barrel growbeds with pipe-within-pipe autosiphons, and a drain that goes through a home-made solids filter into my main drain pipe (down to the sump).


Water is pumped directly up from one of the ponds, using a small submersible pond pump. It has a head of 3.5m. This means that it can pump directly up 3.5m, so it is more than adequate, as the head from the pond floor to the lip of the grow bed is only about 1.5m. It is rated at 3500L/hr and this is important when planning the timing for this pump, because if the big pump is off (which it is for an hour at a time), when this pump is on, it will empty the pond and leave the fish high and dry after only (2900/3500) = 50 minutes. This is what the pump looks like and you can buy one at most nurseries and hardware stores.


I tested it without gravel, and it pumps quite happily to all 6 grow beds. The flow is so good however, that I may need to design a bypass straight to the filter to slow down the flow somewhat. We will fill with gravel and plant with vegetable seed tomorrow (Friday). Here is a photo of the new growbeds all tested and waiting for gravel.


And finally, the first baby tomato made it’s appearance this week, and I’m sure that it will be the first of many, as the bush has flowered profusely.


Enough for now.



4 thoughts on “Aquaponics 101 – Soaking up the Nutes”

  1. 2-3 x-volume gravel to water? At what stocking density of fish? Is there a measurement that covers the amount of fish in the water?

  2. Well of course you could use less fish, but this cuts back on your animal protien output. In lieu of covering your place with tanks (mine have to be all in greenhouses to run all year) you could set up a couple of coral/limestone tanks as trickle filters to convert the ammonia bacterially more efficiently than the grow beds do. Use them in the drainback. The excess nitrate won’t hurt you nearly as much.

  3. Please check my math to be sure I am following this correctly. Rule of thumb – if I had a fish container 4165L I would need a grow bed of about 12,512L or rough dimensions 2.3M length by 1.66M width by 304mm deep. I know the math is a little weird – I am converting to metric – apologies.

  4. After you’ve got the gravel to water ratio correct I just apply a low to medium stocking density for the particular species eg Tilapia 25kg (final) per 1000L of water excluding sump. If your gravel ratio is lower just go proportionately lower on your stocking densities.

    Thanks for your comments.

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