Aquaponic Gardening

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Another TCLynx Public Service Announcement. Or perhaps that should be Fish Service Announcement?

There is a common Aquaponic condition that causes many people to feel the urge to add more fish. This urge should be fought and quashed ruthlessly unless you actually don't have any fish at all.

Just because your system could possibly support a certain MAX amount of fish mass, does not mean that it needs that much. Especially any system that retains it's solids.

During initial cycle up, plants my show signs of deficiency but adding more fish won't fix that, you are just cycling up and the system has not yet gained enough maturity to provide all that the plants need. You will just have to be patient, adding more fish won't speed the process, it generally takes as long as it takes and extra fiddling is more likely to slow things down than speed them up.

Once a system has a little maturity, it can easily keep plants happy with a minimal amount of fish or even run fishless for a while after a heavy fish load is removed.

The more fish you have, the faster things will wrong if anything negative happens. The less fish you have, the more leeway you get if the power goes out or the air pump quits or whatever.

I've never had any of my systems after they were cycled up reach 0 nitrates and I've run systems for weeks and even months without fish. When I have fish I often have a heavy load in the system and therefore, the solids in the grow beds can slowly break down for a long time and will keep the plants happy for ages even fishless. Don't assume your plants are suffering from lack of fish unless your nitrates are reading 0 and the older/lower leaves of the plants are yellowing. (Newer leaves yellowing with green veins is a sign of Iron deficiency, not a lack of nutrients.) I know of many people running systems that have never managed to measure nitrates but as long as the plants are happy, then there are enough fish.

Keep in mind that there are many other things that will make plants unhappy including too much nutrients, too much salt, too much or too little light, too wet around the crown (flooding too deep) or drying out (not flooding deep enough or not flooding at all) lack of or locked out iron because of pH, too high a pH, and many more.

So please stock systems appropriately. Recommended stocking levels would be like 20-25 small fish per 500 liters of flood and drain grow bed (assuming you will harvest them at 500 grams) or 1 fish per cubic foot of flood and drain grow bed (a cubic food is about 7.5 gallons and this is assuming a grow out size of 1 lb so if you are growing channel catfish, make it 1 fish per 2 cubic feet of grow bed.)
Now if you are just starting up a new system and it isn't cycled yet, (Fishless cycling might be a good idea) then you might only want to stock half the recommended level of fish for cycling up with fish.

It is OK to stock less than the recommended numbers!

Now I know many people growing tilapia get away with not following the above guidelines since tilapia seem to survive all sorts of toxic situations with minimal harm but it is still possible to have a fish kill with tilapia if something goes wrong and minimal stocking can work just fine with tilapia too.

Don't be afraid to allow your fish some space. I know those little fingerlings seem so small in a big tank but they will grow, they appreciate the space.


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Comment by TCLynx on October 15, 2010 at 5:21pm
Depends on the type of fish.
If you had really good stock of well started all male tilapia and you have 80 degree water of high quality and you feed a high quality feed three times a day, they can grow out to eating size in 6-8 months. But what do you think mature harvest size is? And tilapia actually show a little more aggression in less dense populations so I don't know if stocking less of them really makes much difference.

Catfish, well they are a bit different but still, you can grow catfish out and eat them in less than a year. We have eaten some catfish we only had for say 6 months before but they were probably only a pound. We usually grow our catfish out much bigger than that.

I'm not sure if stocking less would make them grow bigger or faster. Since we are not really talking about comparing a situation where the fish are so crammed in that they are stunted or something. We are comparing the normal recommended to MAX stocking rates as opposed to what the minimal stocking to keep the plants happy might be.

It is more to the situation that many people find, they don't really need so many fish. In commercial situations, the veggies are the real money maker and the fish are just providing fertilizer and perhaps almost covering the cost of their feed. In which case, why struggle to maintain water quality and remove solids just to keep the fish healthy when it's the plants making the money. If you can spend less money on feed and grow fewer fish to get the same plant output for less work, seems logical to me. And for a home system, the only time you really need more fish is if you are not able to grow enough to feed your family fish as often and they want (and in that case I'm sure you are already planning the next system or the system expansion.)

Growth rates on fish have far more complex factors than just the stocking density in the tank so that question is a kinda hard one to answer quickly.
Comment by Tony Tarantino on October 15, 2010 at 12:29pm
How long does it take for the fish to mature to harvest size? Will stocking less fish cause them to grow quicker and be harvested sooner?
Comment by TCLynx on October 13, 2010 at 11:01am
I've found that if one cycles up fishlessly (I've personally used pee ponics but that would not be appropriate for a commercial system) then there is enough nutrients to start out and then when one adds the appropriate number of fish, it doesn't matter so much that they start small and then grow since there is already a starter boost of nutrients in the system. Granted, I also only really have experience with media based systems and not so much with the raft methods. I think with the raft methods it may be more important to keep a somewhat more constant mass of fish producing waste in the system while a media based system seems to provide more of a nutrient buffer once mature.

It is all a balancing act.
Comment by TCLynx on October 13, 2010 at 8:24am
I know I've been guilty of pushing upper limits on stocking for some of my media based systems on occasion. But we are not eating the fish often enough to really warrant growing so many so other than perhaps a two month period in the spring (when I might arrange to get a large batch of fingerlings to then split with others) I'm going to see if we can reduce our stocking here and try and bring down the nitrate levels in my systems. I certainly have way more than enough nutrients and a far lower stocking density could easily keep my plants happy, probably even happier than they are now.

Friendly has been doing some great work showing how lower density can simplify raft systems.
Comment by M Cosmo on October 12, 2010 at 9:39pm
Well spoken guru. A true Zen Aquapon. Do more with less. This was one of the first aquaponic truth. We all suffer some from the American More more more addiction. Friendly calls it Low Density. I believe that a stocking density of 1lb of fish to 10 gallons is what I am going to strive for.

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