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 Post subject: Why Isn't This Working?
PostPosted: Jun 30th, '19, 03:12 
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We've had an aquaponics greenhouse system running for about a year and a half now, but the system isn't producing nearly at the level we expected it would. I've learned a lot from lurking on this forum, but I think at this point I need to ask for some advice rather than hoping to stumble on the right thing.

The system is a 90-foot greenhouse with four 15x4 media beds with bell siphons and two 60x4 DWC troughs. We have three 300-gallon fish tanks with about 150 tilapia (about 8-10 inches long) and 100 goldfish (about 4-5 inches long). We feed the fish with catfish pellets every other day.

There were a couple of weeks last year in the late spring when everything seemed to be going great and we were getting beautiful lettuce, then during the summer we had great luck with cherry tomatoes. We had crappie in the second tank but replaced them with tilapia in September, and shortly after that the system seemed to fold in on itself. We weren't getting much out, but we put the blame on winter and colder temperatures (we heat with four propane brooders suspended over the media beds). However, the poor production continued on into the spring. In fact, we couldn't even get anything started - the lettuce seedlings we put in the DWC rafts would live about two weeks before shriveling up.

At that point, I started searching through this forum, which made us start testing the water more frequently (I know, we should have been doing that all along....). The ammonia level was 1.0 ppm and nitrites were 0 ppm, but nitrates were 160 ppm - I actually think it was higher than that. We addressed that by backing off on how much and how frequently we fed the fish and by sticking a ton of lettuce in the media beds, where they thrived. The nitrate level didn't really budge, but we did finally get the system back to where we could get some production of lettuce from the DWC troughs.

The problem is that the lettuce just doesn't look right (pale yellowish-green) and we aren't getting much out of it (three or four pounds a week, which is ridiculous). I did some more reading on the forum and figured out that we have a problem with the pH, which consistently measures around 7.6, meaning the plants aren't able to access all those nitrates they need to thrive. We had originally filled the system with water from our well and had topped off with well water. I also learned to test the hardness of the water and found that it had a KH of about 12. So we started collecting rainwater and topping off with it; the KH has dropped to 4. The pH is still staying around 7.6, and the nitrates, although they dipped down to 80-ish ppm one week, are back up around the 160 ppm mark. I figure with enough time and rainwater we might be able to bring the pH down to a level the plants would like better. BUT we've hit the part of Arkansas summer when rain will be iffy, and we know if we top off with well water, we'll just be driving the pH higher.

Sorry for the long setup to this question - Does anyone have advice about what we need to do to get some production going in this system? The media beds do fine - anything we put in them is nice and green and grows well. But the DWC rafts (2/3 of the system) are not pulling their weight. We've talked about revamping the system to convert the DWC troughs to gravel beds, but that would be a lot of work and would take even more money than we've already put into the system.

Thank you! I appreciate all the information and wisdom the forum provides - I've learned more about water chemistry than a former English major would ever expect to know!


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PostPosted: Jun 30th, '19, 04:04 
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How do I delete a post? Our internet service stinks, and it said we had a "bad gateway." I assumed that meant it didn't post, and now I look like an idiot (or a really desperate person!) for posting four times.


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PostPosted: Jun 30th, '19, 11:29 
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Best Guess with info available

So your media beds are fine

Your DWC are not

How are the roots on the DWC plants

The roots need to be white and clean

The water going in needs to be as clean as you can get it

If you have airstones in your raft they could be stirring up the sediment on the bottom

Lift airstones up a bit

We only need a slow flow going through rafts if you have a strong flow it may be stirring up your raft water or carrying fish waste in suspension

All the big commercial systems use DWC so system works fine

Retest your nitrate , high readings can be "Off the Chart" , dilute your fish water 10% / 90% rain water and retest to get a better idea of what you have.

High nitrate messes with plants

We can grow greens with zero nitrate readings and a bit of trace element added

Ph at 7.6 isn't a big problem

Can you post some photos of your system (will need resizing) they often help with diagnosing problems

And we love photos

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PostPosted: Jul 2nd, '19, 10:43 
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In addition to Terra's questions...

- Are you removing solids from the water in the system?
- If so, by what method?
- Where in the system are you doing it?... ie: is all the water filtered before distribution, or just to DWC?
- What are you doing with the solids you are removing?
- What is the water temp in the DWC beds?
- How are you aerating the water in the DWC beds?
- Are you supplementing the system with anything other than what's in the feed, seaweed extract etc,

Also as Terra suggested, can you supply a few pics of the affected plants. Some pics of the whole plants, so we can see whereabouts on the plant it's occurring, plus some close up pics of the worst affected leaves.

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PostPosted: Jul 2nd, '19, 21:39 
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Here are a couple of pictures (attached - I don't know how to insert them into the post).

A little more about the system:

The water from the fish tank goes into a swirl tank that catches the solids. My husband has a shop vac to clean solids off the bottom of the swirl tank, but he hates doing it, so it doesn't get done. From there, the water goes into the media beds, then to the DWC rafts. There's no filter after the water comes out of the media beds. We use coco coir as the medium to fill cups for the lettuce to sit in, and over the past year and a half, a good bit of it has settled to the bottom of the DWC troughs. We've never tried to clean that out.

We have air stones positioned throughout the troughs, and they are probably resting on the bottom. I will loop the hoses and get the stones up near the top of the water today.

Water temp is consistently in the 74-77 F range. We haven't used any other supplements. Also, the water flow through the beds is not all that fast (sorry to not be more scientific, ha ha).

Thank you for your help!


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PostPosted: Jul 3rd, '19, 02:30 
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Judging by the color of the leaves your lettuce has an iron deficiency. Iron isn't very mobile within the plant so the top leaves (the newer ones) turn yellow and the lower leaves (the older ones) are a darker green. There may be other things going on but this is the most obvious. With the current pH, you can spray apply an iron supplement (most types of iron sold for plant application will probably work) or you can get Fe-EDDHA which can be added to the system water. The high pH causes iron lockout for most types of iron - spray applying avoids this. FeEDDHA is available at the higher pH so you don't get lockout when this is added to the system water. It may take more than one spray application to get the color back to where you want it.


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PostPosted: Jul 3rd, '19, 13:52 
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happyhorseshoe wrote:
The water from the fish tank goes into a swirl tank that catches the solids. My husband has a shop vac to clean solids off the bottom of the swirl tank, but he hates doing it, so it doesn't get done.
Those solids contain the mineral elements, ie: Phosphorus, Potassium, Calcium, Magnesium, Iron, etc. You should be collecting them and treating them in a mineralisation tank to liberate as much of the minerals as possible, then add the nutrient rich water back into the system. If you remove the solids from the system without doing this, it will end up with a nutrient imbalance between Nitrogen and all those other nutrients, as Nitrogen is constantly being produced in the system and is soluble in the water.

As far as not removing the solids from the swirl filter, if you leave them to build up for too long they can become anaerobic, then if they are disturbed and that water makes it to the fish tank it can kill your fish. It's best to drain the solids from the filter on a regular basis.

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We use coco coir as the medium to fill cups for the lettuce to sit in, and over the past year and a half, a good bit of it has settled to the bottom of the DWC troughs. We've never tried to clean that out.
Myself, I'd be inclined to use coarse perlite as the media. The coco could be staying too wet, which will inhibit nutrient uptake by the plants.

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We haven't used any other supplements.
As long as your Nitrate reading isn't too high (ie: 80+), I'd be adding a little Maxicrop (maybe the +Iron version) at about 30ml per 500L of water every week. At least until you get the mineralisation sorted and/or the plants are showing good growth.

Also, I wouldn't be worried about getting your pH down. At 7.6 all minerals, including Iron, will be available to your plants. It's only if it gets to 8+ that it's an issue, or if your Nitrate level gets too high, which can affect the uptake of other nutrients. The system pH should gradually decline with time. If you are having to do a lot of topping up during summer, then maybe use the rain water so you aren't constantly adding more carbonates to the system.

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PostPosted: Jul 3rd, '19, 15:50 
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Mr Damage wrote:
At 7.6 all minerals, including Iron, will be available to your plants. It's only if it gets to 8+ that it's an issue


Availability and sufficiency are two different things. Problems with iron uptake can appear well before pH 8. The availability of iron is going to depend on what form it's in. At the current pH and with an obvious iron deficiency showing up on the leaves, mineralizing the solids is a good idea and will provide additional nutrients but I would still supplement the iron until you get rid of the deficiency.


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PostPosted: Jul 4th, '19, 00:37 
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scotty435 wrote:
Availability and sufficiency are two different things.
But are related.

Iron starts becoming "less available" at a pH of around 7.0, and availability tapers off gradually as the pH increases. From my years of personal experience in aquaponics, as well as feedback from, and personally viewing the systems of many others, I can confidently say, here in Perth at least, for our water supply etc, that Iron availability, or lack of, ie: Iron Chlorosis due to high pH, isn't an issue until the pH is above 8.0, usually up around 8.2. I've personally had new systems running a pH of 8.0 or higher for extended periods, ie: 18+ months, with healthy plant growth and no Iron Chlorosis.

Therefore, in regards to sufficiency, it could be assumed that the decreasing availability of the Iron due to high pH, wasn't reducing the % sufficiency of the Iron to a level that was obviously detrimental to the plants until the pH exceeded at least 8.0, or even 8.2

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Problems with iron uptake can appear well before pH 8.
I would suggest the Iron chlorosis would more likely be because of high Nitrates affecting the availability of Iron to the plants, and/or waterlogging of the roots due to use of coco media, and/or possibly even suspended fines clogging the roots (they look okay in the pic though), before it would be a pH issue, at 7.6 at least.

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At the current pH and with an obvious iron deficiency showing up on the leaves, mineralizing the solids is a good idea and will provide additional nutrients but I would still supplement the iron until you get rid of the deficiency.
There is an obvious Iron deficiency issue, hence my recommendation for the addition of the +Iron version of Maxicrop. But again, I would suggest it is not due to the 7.6 pH, but rather one or more of the scenarios quoted above, or possibly even a general lack of Iron within the system.

EDDHA chelated iron, as suggested, is also an option to address the Iron chlorosis issue, but I suggested the Maxicrop +Iron option due to the fact that it contains far more than just Iron, and numerous other macro nutrients and trace elements are probably low in the system due to the solids being filtered out.

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PostPosted: Jul 5th, '19, 01:38 
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For the original poster, for iron at least it boils down to lack of sufficient available iron and this is what needs to be corrected. For everything else I think you should set up some sort of aerobic mineralization of at least a portion of your solids.

I can see where Mr Damage is coming from. It seems to me that if you start out with barely enough iron in a system at pH 6.8 and you raise the pH to 7.6, which reduces the iron availability, then you no longer have enough available iron for the plants. This is one possible scenario for what is happening here. I suppose it is possible that the signs of deficiency won't be visible in a situation like this, I don't know.

The Maxicrop plus Iron addition to the system water makes sense because even though all of the iron you added wouldn't be available, enough could be and it does have growth promoters along with potassium among other things. Maxicrop plus iron doesn't contain any nitrogen so it won't be adding any unneeded nitrogen. My preference is still for spray application, if you spray apply this product it will leave some residue on the leaves. I ran a pH of around 7.8 to 8.2 for two years and used Maxicrop plus iron to supply iron to the plants during this time. I mainly used spray applications but I did add it to the system water occasionally as well. Don't go wild with the addition to the system water, it's probably just a coincidence but there have been occasional cases where the addition coincided with fish deaths (in my system as well).

One of these scenarios probably is what's going on -
There is not enough iron in the system at any pH (you're likely removing it along with the solids)
There is not enough iron in the system at the current pH
The nitrate levels are too high, reducing the availability of iron by affecting the root zone (original post levels were up to 160). I'm not sure this last one could easily occur in a DWC unit where the water is moving.


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PostPosted: Jan 23rd, '20, 05:27 
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An update: We think the problem with the extemely high nitrate levels was not a problem at all - we think the water test kit was no good. We bought a new test kit a couple of weeks after my original post, and the nitrate level went from measuring 160 or more to measuring in the 10-20 ppm range. Go figure.

Since that time, the system seems to have stablized. We've continued using rainwater to top off the system, and the pH has been pretty steady in the 7.4-7.0 range. Recently, it has fallen below 7.0, and the KH levels are low, so we are planning to use some shell grit to add carbonate (and maybe top off with well water every other time). The nitrate reading have gone from off the scale to measuring 0-5 ppm every time, but the plants are looking great, so we're not worried about that. We have more lettuce in the greenhouse than we can sell in our local markets (but that's a problem for a different kind of forum).

We are transitioning from tilapia to all goldfish. Tilapia just don't like our winters, while the goldfish seem just as active as ever. Plus goldfish are so much cheaper and more easily available where we live.

Every now and then, something will pop up as a question, and I'll come search this forum for advice. Thanks, everyone, for sharing your knowledge and experience!


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PostPosted: Jan 23rd, '20, 07:40 
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Thanks for the update. Great to see the abundance. You should start a members thread, showing the system componentry. We don't alway get systems of this size well documented and would really help others. Cheers, Sam


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PostPosted: Jan 25th, '20, 00:00 
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happyhorseshoe wrote:
An update: We think the problem with the extemely high nitrate levels was not a problem at all - we think the water test kit was no good. We bought a new test kit a couple of weeks after my original post, and the nitrate level went from measuring 160 or more to measuring in the 10-20 ppm range. Go figure.

I also had the same problem.The API Nitrate test kept showing one of the red colours.I got a different liquid test & it showed 10-15 ppm and I tried a "dip stick" type & that showed 10ppm.
Now I only use API tests for Ammonia,Nitrite & PH.


I've grown Lettuce at a PH of 7.4 & it grew well.

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PostPosted: Jan 25th, '20, 09:38 
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That is a nice large system! Please do a thread on it! Do you have a YouTube channel? There aren’t many videos of systems like yours. Definitely interests me as I farm trout but want to expand the aquaponics side into more commercial.

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