Backyard Aquaponics
A place to discuss aquaponics

I ran into an article the other night about duck-weed , it explained that duck-weed did more than just use nitrates , it did other conversions that where very good for fish health.. wish I had added the article to my favourites,,, can't find it again now.
So duckweed may be more beneficial than at first thought.

Just wish it were easier to keep lots and lots of it growing. but I don't have that kind of space.

I have to build a raft to stop the fish from eating it. I threw some into the pool over winter when the fish were not eating but as soon as the water got a little warmer.

The half drums are good for growing duckweed. Just have a little system water go through it.

The duckweed did well for the timer we were out of town. Lasted about a week before the fish had eaten it all up.

However, I think it was a big mistake on my part to put gravel in that tank. I'm in the process of scooping out the gravel from the bottom of that tank and I'm hoping the tilapia survive this. See the problem is, it is really difficult to net fish out of a tank with mounds of gravel in it without getting the net full of gravel, Otherwise I might stick the fish over in the quarantine tank while I remove the gravel and let the duckweed system re-settle.

And I am a little worried for the fish in the Duckweed tank because as I scoop out the gravel, the sediments that are getting stirred up are definitely releasing a strong pond bottom smell. Makes me feel I should check for leaches because it definitely reminds me of wading through muck as a kid. Perhaps once I get most of the gravel out, the fish will be easier to net, hopefully I can do it before they get hydrogen sulfide poisoning or something. But they are tilapia and I do currently have more than I need.

Ok, scooped out most of the gravel from the main floor of the duckweed tank. Definitely a pond bottom smell there.

Water completely clouded over with dark muck but I still managed to net out all 11 big tilapia that had been placed in that tank (the 12 was removed some time ago.) I also pulled out quite a few smaller tilapia, some are now in the quarantine tank with the adults and some have become chicken snacks, boy do they love small food that moves!!!!!

Now I'll work on removing the rest of the gravel and then let the system cycled for a while to clear before worrying too much about the stray little tilapia that I didn't catch, I know there are lots of em but I think I was doing pretty good to net the ones I did completely blindly through the muck.

Let this be a lesson to people, aquaponic fish tanks do not need any sort of media on the bottom. Even hides and such tend to trap solids which is usually a bad thing. If you are going to raise any creatures that need sand or gravel on the bottom, probably best to raise them in their own separate tank that can get special cleaning attention if needed.

Granted, Originally I had not planned to add fish into this tank, it was supposed to be for growing duckweed only. However, when collecting duckweed it is nearly impossible to get only duckweed. Leaves and other debris tend to come along with it. If the bottom of the tank has gravel on it, it is very difficult to scoop out debris. Now this might not be such a big deal in a pond with a huge flow of water filtering it and/or huge aeration but the duckweed peeponics system is running on less than 30 watts of power.

I think that system is doing darn well to have supported so many fish on so little power.
Once cleaned up a bit, I'll probably put some tilapia back in it to breed up my overwintering stock since I want to start with small fry as late in the season as possible or they will get too big for the aquarium before it is warm enough outside for them.


Ok back out to finish scooping out the smelly gravel. (Not terribly bad smelly -not like rotting fish food that makes me want to puke- just kinda like pond bottom smelly.)

Hum, there appears to have been an extra large fish in there than I hadn't realized was there So now there are 12 large tilapia in the quarantine tank and untold smaller tilapia in there as well.

Been scooping out gravel, hard on the back, worse than gravel washing I tell ya! Starting to run out of places to stow the gravel. I want to make sure it is in containers that will drain as I don't want to promote the anaerobic bacteria but I want it convenient for the next idea I come up with that needs some gravel.

either the smell is already starting to dissipate or I'm getting used to it.

Ok, aching back. Definitely worse than washing gravel. Trying to scoop gravel out of a tank when the bottom of the tank is several inches below ground level and most of the space isn't quite right for kneeling next to the tank for the scooping. At least the one end of the tank that had the most gravel, I was able to remove the walk boards over the tank and kneel next to it to scoop there. And then toting the gravel to a place to stow it for the time being.

I don't care, even if you don't think you will put fish in it, don't put gravel or sand in the bottom of a tank unless you have a really good reason to do it (like maybe for yabbies or muscles.)

I think duckweed is a wonderful thing. I found that when I had that tank full of duckweed along with 12 adult tilapia and untold numbers of fry and fingerlings, I could feed quite a bit of food to them and the system somehow was not overloaded. However, that same system without the duckweed was limping along to support those fish with any sort of heavy feeding.
Granted, we are talking about tilapia here which are pretty bomb proof fish but duckweed seems to be a really good filter. Put it this way, the duckweed peeponic system only has about enough gravel to support 10-12 lbs of fish properly. I mean it is really only a barrel ponics system hooked to an additional long narrow almost 400 gallon tank. The only power that poor little system is getting is the little pump having to lift over 6 feet so I'd be shocked if it is doing 100 gallons an hour. And a little two outlet air pump that would be appropriate for a 60 gallon aquarium.

Dang I need more duckweed since the fish ate it all while we were away.

By the way, I got most of the gravel out of that tank. Water was clearing enough today for me to see the many little fish still in there. Unfortunately the sediment re-settled instead of moving on to get pumped into the grow beds. I might have to pull another pump out to see if I can move some of that sediment out of that tank.

My little buggers (five, seven and under) help put gravel and sand in my FT. They now know not to! I was able to use a fish net to get the majority out, but I don't envy all your work TC! In your opinion does duckweed help keep the algae down, since it blocks most of the light on the top? My system was going along great until my nitrates went over 50ppm, then bang, the water clouded up and there is algae everywhere (lost a few goldies, too)! I got lazy and didn't check the nitrates seeing as it didn't really mater. I guess the tomatoes and peppers weren't pulling their weight!

Nitrates in the middle of the scale like that are fine for aquaponics but it usually means that we have to keep the fish tank completely shaded or algae grows to compete with our veggies. And of course the issues with algae messing with pH and using up all the dissolved oxygen in the wee hours of the morning and killing the fish.

Now that my system is well established, I don't have to be as careful about keeping all light out of the fish tanks but I still keep dark fabric over the majority of the tanks (at lest the ones protected from the rain.)

As to the duckweed keeping algae down, well yes it does, it will also steel nutrients from your veggies though. I don't know that it keeps enough light out to stop algae but it does use up ammonia and nitrates so in effect it uses up what algae would need. I'm not sure if filling a algae filled tank with duckweed would work to get rid of the algae because I don't know how well those two compete but once you mange to clear a tank of algae, you could fill it with duckweed to perhaps keep the algae from coming back. But duckweed doesn't like bright direct sun so much and depending on the situation, duckweed might not be an improvement on algae since both stop you from seeing the fish and the bottom of the tank.

I do think it would be possible to have a fish and duckweed system where the fish were in one tank and fed some duckweed as well as other foods and then the water gets some solids removed as it goes to a duckweed bed (would be much like a raft bed but minus the raft since the duckweed floats) If one wasn't trying to raise huge amounts of fish and didn't need more veggies, I'm thinking this might work. Just scooping off a net full of duckweed to supplement fish feed every day but other wise keeping the duckweed bed fully covered. would be a good system for a shady area since duckweed doesn't like too much bright sun.

Alright, duckweed tank cleaned out.
I drained it down and it was a chore catching all the small fish. Chickens got a good high protein feed today!
I finally stuck a sump pump in it to help me wash out the fine sediment.
Now there is only a trace of the sediment in the big duckweed tank and just a small amount of sand and gravel lurking about.

Re-filling with well water and will run for a bit (perhaps give it a humonia dose now that it's cleaned up.) (It still has some of the original water that has been cycling around in the barrel system-pretty easy to move two siphon pipes and the barrel system can run without the duckweed tank.) and of course the gravel is all very well seasoned.

Then perhaps after we get some more duckweed going, perhaps I'll put some tilapia back in there to breed up my overwintering fingerlings.

TCL,

About how many square feet of duckweed growing area does this system support? I'm trying to get an idea of how much space it would take to properly supplement the diet of a few tilapia and I can't remember if you mentioned the total surface area (it's been awhile since I read through this thread, so I apologize if the information has already been posted).

I can't remember exactly how wide the duckweed pond is but perhaps almost 2 foot wide by about 16 foot long. If some one reminds me in a few days I can go out and measure but at the moment I'm on the other side of the country.

Duckweed can support fish, however, what are you using to support the duckweed then becomes the question. I've found that water with a nice trace of ammonia seems to support duckweed quite well but a pretty big surface area is needed to support more than a very few fish. As noted before, that surface area could start out with a good thick layer of duckweed and 12 fish would clear it of duckweed in under a week if they were not being fed pellets daily.

I'm sure if you do some internet searching you will find some sites about duckweed and how much can grow per some given amount of surface area but I seriously doubt that you will grow fish to eat without adding outside feed of some sort. (either fish food pellets or a separate system with fertilizers to grow enough duckweed to feed the fish.)

Wow, I was pretty remiss in not updating this thread ages ago.

I have discovered that it is difficult to keep a good duckweed tank going because the debris that gets in settles to the bottom, that along with any solid wastes can create a nasty situation in a slow moving still water tank. As soon as that stuff gets stirred up, it can tell a tale of hydrogen sulfide and really stink.

I now think a duckweed system needs to be very well protected from any solids or debris, otherwise it could spell disaster for any fish in anything with a connected flow.

This past fall we harvested all the larger tilapia and gave away many smaller tilapia in prep for the cold. I brought a dozen small tilapia into the aquarium in the house.

The rest the the tilapia that were difficult to net from the duckweed tank, became much easier to net when it got cold and they died.

Then the barrel system with the duckweed tank cycled fishless for the rest of the winter.

After code enforcement made me take down the plastic and the roof over my main system and all. I decided to use the barrels I had and the barrels from the barrel ponics set up to create a nice line of barrels growing bamboo to help screen the front corner of my property. And since my monster grow bed on the big system was leaking, we made a big change and the gravel from the monster bed went into the barrels as well as into the duckweed tank. The duckweed tank now has half of the gravel from the monster bed and the pump from the monster bed.

The duckweed system is no more.

I think u've convinced me to try and setup a small duckweed setup to try and pull out some of the ammonia and eventual nitrite that are hurting my fish (my kids unplugged the whole system for 7 or 8 hours and 1 fish so far is dead).

Also, i've wanted to ask this earlier, why do you try and keep the PH so high? Watching some of Murray's dvd's he seems to think that 6.2 to 6.4 is about perfect.