Ok here is another one of those questions I’ve answered many times and this time when I found myself almost typing out a long post again I decided I should do it on my blog to make referring back to it again much easier in the future.
Often the question is like, How many square feet of duckweed do I need to feed some number of tilapia?
I once set up a duckweed tank. It was 16 feet long by about 30 inches wide and 18 inches deep. It had shade cloth over it and started out as part of a pee ponics system hooked to a barrel ponics system. The pump was down in the barrel that was set a few inches into the ground so the top of the barrel and the top of the duckweed tank were about the same level. The pump was in the barrel and lifted water to the top bed, then it went to the flush tank and into the half barrel beds which actually drained over to the duckweed tank and the far end of the duckweed tank drained back to the barrel tank. System was running on a 30 watt water pump and a 5 watt air pump.
The Duckweed definitely seems to grow better in aerated water though it doesn’t like much surface agitation or water level fluctuation.
Duckweed can use up a great amount of ammonia but if it dies off, sinks and decomposes it will give off much ammonia and other nasty stuff if the decomposition goes anaerobic. Also duckweed can clog filter screens, pump intakes drain grills etc so keep that in mind. Once it is in a system it tends to get everywhere. Unless of course you are trying to grow it. See Duckweed seems to grow best as a weed. What does that mean? Well a weed is simply any plant that is growing where you don’t want it. It seems to thrive where you don’t want it and when you do want it, it doesn’t seem to do as well. That is just my experience, ya have to trick it if you really want to grow it.
Anyway, So I had this duckweed tank and we gathered wild duckweed and kept it in this separate system for a while. Then I decided I would put some tilapia in the duckweed tank. I put 12 tilapia between fingerling and breeder size into this 16 foot by 30 inch by 18 inch deep tank covered with a thick growth of duckweed. I was feeding the fish daily (over by the aerator where there was a circle of clear water to drop the feed in.) The cover of duckweed remained good. This is actually problematic since you can’t see the fish at all under the thick cover of duckweed and trying to catch fish you wind up with nets full of duckweed and it covers everything including you.
Then I had to go away for a week and I didn’t have an extra automatic feeder to hook up for the duckweed tank but I figured they would survive under the duckweed. Well they did survive but by the time I got home there was no duckweed for them to be under. So that was less than a week for 12 small to medium size tilapia to clean up almost 40 square feet of a thick growth of duckweed. But if they were not starving, I don’t think they would have eaten much of it at all.
Now this was in our WARM part of spring here in FL so those small tilapia had started breeding and I had lots of small fry/fingerlings growing in that duckweed tank after a while. That was a very effective breeding tank for my tilapia but netting fry/fingerlings from the duckweed is even more problematic than netting large fish from a duckweed covered tank.
So, how much duckweed do you need to feed tilapia. Well I would say you need at least 40 square feet for each dozen tilapia if you are feeding only about 50% of their diet as duckweed. I would say the duckweed should be grown in a separate system so you can fertilize it, otherwise you don’t get much nutrients available for your Aquaponics system.
However, if your not running aquaponics and you need an ammonia sink, then perhaps recirculating water from the fish tank through a large shallow pond plant bed in a shady location and a little aeration will grow a good amount of duckweed that you can then scoop off and give to the fish regularly.
I do not think duckweed will provide a huge benefit of aeration to the fish tank since by covering the surface of the water the duckweed will impede a good amount of the natural passive aeration available to the fish tank so during the day the duckweed may break even on the amount of aeration available to the fish tank but at night, the duckweed will more likely produce a loss of oxygenation to the fish tank since plants only give off oxygen in the light. And if there is a die off for some reason, the duckweed like algae decomposing in the water can quickly increase the biological oxygen demand in the system and rob it of oxygen leaving your fish to suffer. So while you might be able to manage a low energy fish/duckweed system, I would definitely recommend keeping aeration going especially at night. Since flood and drain is NOT desirable with a duckweed system, one could set up a minimal height variation system using only air to provide flow via airlift pumps and reduce the problems with pump clogging due to duckweed covering pump grills (when duckweed dies the roots sink and can cover pump screens with a fine hairy slime.)
Duckweed is a fine thing but it is not the miracle some people get distracted into thinking it is. Take into account it’s failings and challenges before getting too enamored with it. It is not a complete feed for tilapia and can only replace about 50% of a commercial pellet diet before you will see loss of growth in the fish. If you are not trying to grow it, it seems to do far better. When you are trying to cultivate it, it seems to be more picky about conditions. It uses ammonia directly so is good for a system running short of filtration but if you are trying to grow veggies in your aquaponics system, growing duckweed in your aquaponics system will be robbing your veggies of nutrients and by using the duckweed as fish food you won’t be putting enough nutrients into your system in the first place.