Confused about fish density when just getting started

[David Stiller]

On page 134 of Aquaponic Gardening, it says, as a rule of thumb, to aim for 1 pound of fish for every 5 to 10 gallons of fish tank water.  If I go with 1 pound per 5 gallons, that means a 150 gallon tank should handle 30 1-pound fish.  Maybe a handful fewer, because I'll probably have less than 150 gallons in the tank, right?  (Wouldn't 150 be filled to the brim?)

This all makes sense to me when the fish are full grown, or nearly so.  I'm confused, though, about how to stock my tank at the very beginning.  I'm assuming I'll buy fingerlings (probably tilapia), but fingerlings weigh nowhear near one pound.  Do they even produce enough waste to feed the biofilter?  In other words, can I start growing plants right from the get-go, or do I have to wait until the fish reach a certain age/mass?

Page 135 features a callout with advice by Dr. Lennard, which says, "beginners are better off ... having patience [with small densities] than going for a high fingerling numbers and then trying to get rid of them later."  That's comforting, certainly, but it still leaves me unsure of how many fingerlings to acquire.

I think I'm supposed to gauge my stock density based on estimated fish weight at maturity, but then I wonder, do I have to wait several months before I can start growing plants?

[Jonathan Kadish NYC AA Chair]

It would help to know how big your grow bed is going to be - this will be a determining factor of your stocking density. Most people think about the fish first when they are starting out while you need to be just as concerned with what you are going to do in your grow bed. The primary reason that we have fish is to feed the plants, otherwise we would be doing aquaculture. You can grow a lot of plants with just a few fish.  Also, you can start growing plants even before you have fish when you begin to cycle the system. Read the chapter on fishless cycling.

Your getting there, aquaponics is actually fairly simple if you are not pushing the boundaries.

[David Stiller]

Johnathan,

Thanks for the follow up!  I'm strongly considering a 50-gallon tank as my grow bed.  Rubbermaid makes 50- and 150- gallon tanks with approximately the same dimensions, except one (the fish tank) is two feet tall, while the other (the media bed) is only one foot tall.  In rough terms, the grow bed would be 3 by 5 feet.

[Jonathan Kadish NYC AA Chair]

You want to think of a 1:1 ratio in volume as a starting point. You can go up to 1:3 adding more grow beds but going the other direction of more fish to plants will result in ineffective filtration.  So if you are planning of filling your 50 gallon tank with gravel or hydroton you would only want to stock your 150 gallon fish tank as if it were 50 gallons.

[David Stiller]

Jonathan,

Wow, that's an eye-opener!  That isn't how I interpreted the ratio from the book at all.  On the one hand, what you're saying makes sense, because the media bed holds 50 gallons.  But then, most of that space (or a lot of it) will be taken up by hydroton, right?  So it won't be 50 gallons at water at all.  I'm probably making this more complicated than necessary, but your interpretation calls for one third the fish I was going to put in my tank -- that's a big difference!

[Jonathan Kadish NYC AA Chair]

Look on page 70 where Sylvia talks about volume relationship between grow bed and fish tanks.

[David Stiller]

I'll definitely look into it again.  Thanks!  Clearly, I was tripping over the words.

[TCLynx]

The numbers given of 1 lb of fish per 5-10 gallons of water is more about what the max grown out fish density is.

And I agree that you want to make sure you only stock as many fish as your grow beds can filter for.  So if you are growing fish that might reach a pound each, then you should start out only stocking 1 fish (for each pound of fish that you can handle max.)  Remember small fish often eat higher protein feed and eat more for their body weight so they will produce more nitrogen rich waste to support your plants even though they are smaller.

Ok, to figure out how much fish your grow beds will support.  Well for a newbie that still needs to ask how many fish, a good number would be stock one fish per cubic food or grow bed media (and if you don't have nice rectangular beds that are a foot deep and easy to figure in cubic feet, just figure 7.5 gallons of media is about a cubic foot.  So a 50 gallon grow bed is about right for stocking 6.6 fish (if they are fish you would grow out to 1 lb each.)

Now if you do fishless cycling, you should have ample nutrients to get the plants going from the beginning.

If you are cycling with fish it may take a little longer until the plants will really have enough nutrients to take off so the first crops for a system cycling with fish should be the less demanding plants.

[David Stiller]

TCLynx,

Thanks for the additional notes!