Aquaponics - Fish stocking density


1 fish @ 500g per 20L of filtration, or 2.5% of the media in fishmeat by volume (fish being neutrally buoyant equal water weight by volume  (1 ml of water = 1g) seems like a reasonably common stocking level.


So for every 20 L of growbed media (gravel, clay balls etc) you can stock 1 fish that you intend to grow out to plate size.

Plate size is considered to be 500g, and reflects what a restaurant might like to serve a customer on a plate rather than the actual size of your plate.

I dont know what size your plate is.

Although it's probably, by coincidence, roughly the same length as a "plate sized fish" wide.

Or high.


So a fish, that looks nice on a dinner plate, is around 30cm long, weighs around 500g, or a little over a pound,  and requires around 20L or 5 ¼ gallons of filtration or grow media to support it throughout it's life of pumping fish crud into the water.

“Stop eating so much. You don't need that much protein in one meal.” I sometimes tell myself. But I'm wrong. Fish is delicious, so I'm probably going to keep eating that much.

So you put a stack of fish into your system, and you end up waiting quite a while, then you pull them all out, and put them in the freezer.

But fresh is best.

Why don't we eat smaller fish? Fish are crazy brave when they are young, and feed like mad taking all kinds of risks to get to the food before their fellow fishies.  This means they grow quite quickly when they are young.

Trout and barramundi seem to grow to plate size in 8 months. But that might be because they are already quite grown up when you get them. Silver perch take around  two years. Or actually two summers, as they dont feed a lot during winter. Most fish varieties grow quite fast at their preferred temperature.

So if our systems need the number of fish they can support to give the vegies their best conditions to impress, why do we have so few fish for so long.

Most people stock a number of fish that their system can cope with once they have grown to plate size. But that means the system is low on nutrients for the greater part of a year, and then perhaps overloaded for a bit, then suddenly, has no nutrients for the plants at all when the fish are all harvested.

The system's resident veggies must hate it.

But what this means is that you either have to supplement your ammonia, or nitrate inputs into the system with Charlie Carp (fishy goo) or something to keep the plants happy.

So, all that stuff is true.

But its also true that we eat fish that are smaller then a plate sized trout. Sometimes a lot smaller.

In South Australia, where I happen to be, we eat Australian Herring, Gar Fish, Leather Jacket, Yellow Fin Whiting, and almost every other fish we eat can be bought or legally caught at smaller than plate size. I think even our King Gorge Whiting, considered by many as one of the greats, are legally caught at less that “plate size”.

So what's so good about plate size?

Don't answer that, it's a rhetorical question. Unless there is actually an answer...

then …

go for it.


Ideally we should stock our systems with 100% of their fish meat holding capacity, and start eating the biggest of the small fish the following week.

That could get a little finicky when trying to fillet a 4cm fingerling, but perhaps there is some kind of compromise.

Silver perch take two summers to reach plate size.

Perhaps we should stock an amount of silver perch, such that after one summer, there is enough fish by weight, that we are not over stocked, but that we can start eating. They grow slowly in the colder time, but that might mean we can slowly eat some in an attempt to keep the stocking level at close to optimum, and when the next summer comes, we still have the right amount of fish, but we can start eating more, more often, until we find ourselves with one last megafish, still capable of running the system.

Obviously rather than one mega fish, it would be better to buy more fry at a time when the system could afford ...say... 50 new fish, if there was one less big one in the system.

That should be the trigger for buying new fish.

After working out how many fish such a program would require to restock, approximate the big fish equivalent to the number of new fry, and restock when eating the big fish would allow enough filtration media, to buy a new batch of small fish.

This might seem obvious to some, but it doesn't seem to be normal practice.

Given the price of decent quality, ethically raised, organic, un-polluted, un-heavy metalled, fish, and the feed conversion rate of around 1:1.2 (ie 1.2 kg of feed makes around 1kg of fish (insects, algae etc make up some feed, and fish do a whole lot of floating perfectly still waiting for food to wander past them, so they are fantastically efficient(some trials have shown better than a 1:1 ratio))) …

where was I...

Given all that, and the fact that you pay a bit for your new little fish (around $1.80 for me) it still works out to be an exxcellent deal to eat the fish way before they are plate sized.


I think I should try to work out how many fish I should buy to make this form of stocking a reality.

120 Things in 20 years needs to make a spreadsheet to work out a better aquaponics fish stocking density plan. Or just take a bit of an educated guess.


  1. What about 3 fish tanks as part of one system.
    Tank 1 for first year fingerlings,
    Tanks 2 for 2nd year fish,
    Tank 3 for plate sized fish.

    Catch and eat the fish out of tank 3. When it's time to order new fingerlings, catch and freeze any remaining fish in tank 3 and stock with the fingerlings.

    Tanks get renumbered at this point.

    Fresh fish year round AND frozen fish for them lazy days when you don't want to go out in the rain to go fishing.

    And, they can be bigger than plate size. Although, this may incur more capital expenses in purchasing larger plates.

    1. Silver perch of all different ages can (I'm told) all live in the same tank. But my big fish was such a bully that I wouldnt do it.

      But that's what the original system was to be (fish tank for big fish, sump for fry), but I'd rather not have to freeze anything.

      I'm getting a little obsessed with fresh is best at the moment :)

      The most important part is if you're stocking with trout, start eating them at 4 months rather than 8, it means you can get a lot more fish out of your brand new system.

  2. Doing it the way I'm talking about still requires more than one tank, because you are restocking when you still have large fish.

  3. What you need is a fish tank built on top of a scale. Then calculate the total weight of fish your system can support, plus the weight of the water and tank. Stick fingerlings in until the scale reads that weight. Then feed 'em until the total weight is the ideal weight plus a meal's worth of fish. Catch fish and take them out until the system is back to ideal weight again. Eat 'em. Feed the rest until the total weight is ideal plus meal again, repeat.

    1. I am not sure that would work because the fish are approximately the same density (after swim bladder) as the water so there isn't any net change in weight of the system at a given fill level.

      We just need to put a chipped plate at the bottom and when the plate disappears, scoop whatever's on top of the plate out!

    2. Dang, I forgot to take the constant height of the water into account.

      Ah, well. It would have been hard to find a scale that big, anyway.

  4. This comment has been removed by the author.

  5. I think I'll rebuild the demand fish feeder from scratch, and add an "eat a fish now" alarm function.

    I'm pretty sure the invention machine will make it work. It's just the kind of thing it's good at.

    But then I would say that wouldn't I :)

    Oh, and I think Mike's probably right about the weight thing.


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