Duck weed is a water plant so it won't take over a gravel bed (the way something like mint might.)
However, there are some people who ran into some major problems with Duckweed (Friendlies) so beware.
The trick is, in a heavily stocked system with places where duckweed might lodge and die and turn into a rotting mass before you notice could steel all the dissolved oxygen and kill fish. So, make sure you can monitor closely in any system where you are growing both duckweed and fish together since anything rotting en mass can use up an astounding amount of dissolved oxygen.
In a raft system, if some gets into the raft beds (which it is really hard to keep duckweed from traveling a system) it will be difficult to totally get rid of. However, duckweed likes fairly still water so if you have aeration in your raft beds, the duckweed will be limited.
That all noted, we have some members who grow lots of duckweed and it is a great supplement to commercial feed for fish like tilapia.
Duckweed is a great ammonia sink, what I mean is duckweed uses ammonia directly so it would prefer to get your system water before it has gone through a bio-filter and it can use up a huge amount of ammonia so if you have a system where the bio-filter is having a hard time keeping up with your fish load, then perhaps a settling tank/duckweed bed before the water moves on to the bio-filter might be worth while.
Duckweed will clog screens and filters and grates so I recommend using SLO drains or venturi drains on any tanks growing duckweed so you can avoid overflowing the tanks when the screens clog with duckweed. Draw the water out from below the surface since duckweed floats.
I don't know. Duckweed survives in climates where there is real frozen winter so it must have some way to start up the next spring after winter so I don't think there is a guarantee that the duckweed wouldn't survive freezing.
Duckweed can be dried and then it could be fed much like dry flake food.
It can be grown remotely(out of the recir. loop), this way you won't subject your AP to any problems which may arise. This way it won't affect the system nutrient balance and you're better able to control the population. Stagnant water, dead rotting tree bark or branches, dying leaves from AP crop etc are what's needed for growing it. You can top up with system water or small amounts of fish feed periodically for increased growth. Doesn't require much space so a small vessel will do.
Chris McMahon said:
Interesting. I am going to grow some and see what happens. I will run some tests and see what I find out.
Chris - duckweed normally grows in standing ponds with a lot of dead organic matter in it where it is adapted to grow rapidly using ammonia rather than nitrates - thus getting an edge on most other plants. It usually reproduces asexually which acounts for its rapid growth, but can survive poor weather / low water conditions by reproducing sexually and leaving dormant seeds waiting for better days.
I know that the Friendlies have publicised their woes with the stuff but I'm not sure what went wrong there. I am running a system filled with it and have never seen the kind of issues they referred to. As TC said, if you have a system with remaining ammonia, the duckweed will suck it up. I currently have 38 tilapia (large) and 17 koi overwintering in a system that only has duckweed in it (7 square meters) and the ammonia stays below 0.25 mg/L. The nice thing about such a system is that it can literally be set up in one day. No cycling needed as you are simply feeding the duckweed ammonia and pulling out the solids with a swirl filter.
Vegetarian and omniverous fish devour it, but it can throw you a curve ball in terms of determining the amount of nitrogen added to the system. 85% or more of the plant is water and its protein content is highly variable.
I have used duckweed regularly to supplement the diet of our tilapia for many years. If the system is designed properly it will not clog. The fish love it.
One could probably do a duckweed system almost like a green water system where it was just duckweed pond fish and fertilizer. I don't think there would be too much nutrient left over for veggies and it would probably not be a terribly fast way to grow fish but more like extensive more low intensity aquaculture.
On the other hand, I've sometimes through about hooking up a duckweed tank temporarily to a system that is experiencing and excess of nutrients. Would want to send the raw fish tank water to the duckweed and then use a SLO drain from the slow flow duckweed tank and after the duckweed one might want the clairifier or settling tank to collect any excess solids from the duckweed that have sunk. This sort of layout will require occasional cleaning out of the bottom of the duckweed tank of the settled fish tank solids.