Backyard Aquaponics
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I’ve had my DWC system up for about 6 months and I’ve been battling green algae for the longest time. I have 4 half 55 gal barrels as raft beds and two 55 gallon totes as fish tanks with about 10-18 2” goldfish in each. For filtration I have two small plastic pots over my bed outputs with polyester and bio balls underneath. Going from fish tank to raft bed I have two solids filters with netting in each for solid collections. Since I’m on solar, my pumps run 30 min off 30 min on all day and at night I have air pumps
On all night. I have not feed my fish in about a month and Im not sure what else to try. Not sure if it’s just part of the long cycling but any help would be much appreciated!!

Thank you!!


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For a start block out most of the sun light on the water

Sorry to hear you're having troubles. In many cases with media based AP systems the algae will just disappear as the phosphorus goes lower - I doubt you'll be so lucky since it's still around after this long. Most solids filtration really doesn't work too well on algae so I don't think that will be a solution even if you increase the amount of this you have. You didn't mention where the algae is a problem (DWC, fish tanks or both)?

FWIW - it's best if the algae doesn't all die at once since this could cause an ammonia spike.

There are several things you can do.

1. Block light getting to the water. Most blue barrels allow light in through the sides so it may not be enough to block it from the top.

2. Add more plants to the system. Phosphorus is usually the limiting nutrient that allows rampant algae blooms and by having more plants you can get this down enough that the algae won't do as well. You can also use floating plants to soak up some of the excess nutrients (water hyacinyths, duckweed or azolla for example), then scoop them out to a compost pile to get them out of the system. Duckweed and azolla might provide some chicken feed if you have chickens.

3. Change feeds to one with less phosphorus. Sometimes difficult to tell how much is in some feeds and this is the least likely to be an option since it seems wasteful to not use up what you have.

It seems like without making any additions your plants in the DWC shouldn't be doing very well, is this the case or are you adding nutrients in some other way?

Edit: Hi Milne . Good to see you're around

Hi thank you for your response!
I was thinking of making a shade tent over the beds, do you think that is a viable option?
I already have plastic bags over the fish tanks.


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Hey Scotty! I have the Algae in both dwc and fish tanks. It sucks cause I had it under control and the plants were doing well then I tried to add some chelated iron, calcium and potassium by dissolving it and spraying it. I don’t think that’s what caused it to come back. I have a feeling there’s “too much sunlight” because its in a greenhouse.
I’m using tetra gold fish flakes but like I said before I haven’t fed them in a while because of the nitrite issue a couple of months ago. The params are all good except PH which is 7.8-8.0.


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Try putting a few koi in there, they do a good job in my system, if you put any in you DWC they ,might eat the roots off your plants.Mine eat the aigae off the side of the tank.I can put heaps in there from another tank and it is all gone next day.

Try blocking the light as suggested. I'm a bit concerned about the plastic bags over the fish tank - not blocking oxygen transfer if the pump goes out I hope? You'll actually get less light in the greenhouse because of the film but it'll be nice and warm for the algae to grow.

Bet you still have measurable nitrate levels after two months of not feeding.

I took away the plastic bags but they were never in a spot to inhibit O2. I checked the Params but everything is good except PH (7.8-8). You think the algae is soaking all the nitrates to make my readings near 0? Are there ways I can chill the water? I hade duckweed in the dwc and fish tanks but they won’t grow at a good rate.


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Actually I was thinking you still have excess nitrate but the algae could certainly have used it all.

Yes there are ways that you can chill the water but probably not worth your effort unless you have trout or another fish that would benefit from it. Some people use solar and can run chillers at fairly low cost. Another way if the temp is high enough is to use evaporation - something like a bakki shower or trickling tower. Doubt it will help with the algae enough and will slow your regular plants down as well. FYI - if this is your first year with a greenhouse you might need to put shade cloth over at least a part of the greenhouse during summer to keep temps down to reasonable levels inside.

If the duckweed isn't growing then you might be better off getting some shade cloth or building a cover for the fish tank that lets air in but blocks the light. If there are holes that aren't being used for plants in your DWC or gaps in water coverage then probably should block the light through these as well. Post up some photos of your setup if you can, that will give us an idea of what needs to happen .

I'd definitely put something in front of the tanks - maybe just let a piece of cloth hang down in front. I'm not sure about your plants - you might need to restart with some fresh plants. Sometimes they get stunted and there's nothing you can do to bring them back. Tough to tell in this photo but looks like they may have an iron deficiency - leaves look pretty yellow. Could also be nitrogen if that's been low.

Algae is tough, and you may need to empty out your system, scrub out the algae, and then block the light, as recommended by others, to prevent its return. I would also start feeding your fish bit to keep them healthy.

I found a weather tarp I had laying around the shed and put it on, the past 4 days algae has reduced significantly in the right fish tank. I’ve been leaving air stones on 24/7 for algae die off. The left side still hasn’t changed and I think it’s because that side gets exposed for longer.

This is a pic I took today of the tarp setup

Hmm. You are mainly trying to block the light from any open water (to knock down the algae), although, sometimes shade cloth is of benefit for the plants as well (the ones you're trying to grow). Greenhouses frequently get too hot in the Summer and so shade cloth would be needed. The tarp might help a bit as long as you aren't cutting off too much light to the plants you're trying to grow.

You might need shade cloth on the greenhouse but you definitely need a cover of some sort for the fish tank. One that lets in air but blocks the light.

FWIW, If I'm growing plants in the greenhouse, I use shade cloth when it gets hot in
summer. Before I was into AP I used shade cloth starting in spring. Having all the water mass inside the greenhouse helps even out the temp in the greenhouse so it doesn't get as hot as soon as it used to. I would tie mine to go over the top of the greenhouse over the outside but it can be used inside - My setup was simple and worked but was not as versatile as what commercial operations sometimes do.