Stuppy Aquaponics Blog

Mg deficiency solution and Clarifier Design for Aquaponics

Posted by Conor Quinlan on Nov 7, 2016 10:39:36 AM



Nitrogen (mg/L)






Total Ammonia




Temp (Fo)



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By: Conor Quinlan

Water Quality: I have decided to push the parameters of the system again to observe how low the pH can get within a given feeding range. I have increased the feed amount from 150 grams feed per day to 240 mg feed per day over the past week and will not add CaCO3 to increase pH until pH falls around 6.4. The increased feed and lower pH should also help the magnesium deficiency that has been observed in basil crops. Nitrification is proving to be more than sufficient, even at these lower pHs, thanks to the large surface area of the media bed. Now that the feeding rate has increase substantially it will also be interesting to see how nitrification performs over time.

A new, far more accurate, hardness test that uses titration as the chemical testing method was used to determine a more accurate estimate of the Mg concentration. The Mg concentration was lower than previous estimates however with a concentration of 55-67 mg/L that should be more than enough. Magnesium Sulfate was added to the system in very small amounts, only 25 grams for a total Mg concentration increase of 2 mg/L, to quicken the reverse of the Mg deficiency. This addition should help speed up recovery as the increased fish feed is processed and converted to nutrients.


MAGNESIUM PROBLEM AND HARDNESS TESTING: I conducted a far more accurate hardness test using a titration method. I have used the strips before and although they provide decent accuracy for lower hardness levels I recommend using only titration based testing methods for aquaponic system water. Previous tests with the strips indicated that hardness was around 900-1000 mg/L but the new tests indicated those levels are closer to between 675-700 mg/L. This decreases the estimated Mg concentration from 100-125 mg/L to 55-67 mg/L, however these concentrations are within the proper range for hydroponic production of herbs like basil.

 It is possible that the Mg is being made unavailable to the basil somehow but any cation competition is unlikely because the concentration of K and Ca are not excessive. The most likely cause at this point, in my opinion, is that the smaller water volume of the media bed makes nutrients less available on a per crop basis. Currently, due to the buildup of waste in the media bed, it can only hold about 40 gallons of system solution versus the float bed which holds nearly 300 gallons. So within the same plant area the media bed receives only 13% of the nutrients available in the float bed.

This means the media will likely have to be clean up soon to clear out waste and clogged media to provide more space for incoming nutrient solution. Luckily most of the media can be reused, how much though will be analyzed during the cleaning. This is another major reason why having a clarifier is important and that will be the next addition to the aqueduct coming soon.

New Information:

CLARIFIER: The new 30 gallon conical tanks that will be designed into a special radial clarifier have finally arrived this week. By the end of this year the clarifier will be added to aqueduct package and will provide filtration of the settable solids produced by the fish. The solids that collect in the clarifier can them be drained from the system and re-used as a soil based fertilizer supplement.

Individuals who already have the current aqueduct model will be able to purchase the clarifier as an add-on. Below is an image of the clarifier tank.

Clarifier Tank.jpg

Plant Quality: All of the crops besides the basil are growing with no issues to report. Exotic species like the artichokes and asparagus appear to be growing very well thus far. The jalapeño pepper crop also yielded over 25 peppers from a single individual yesterday when harvested.

 The fact that the media bed is not receiving enough nutrient solution per crop individual and that basil is sensitive to lower Mg concentrations are likely both contributing to the Mg deficiency seen in many of the basil individuals. For this reason, in the near future, I will be transplanting a deficient basil crop into a 5 gallon bucket (equal to the water volume per crop in the floating bed, 300 gal/60 crops = 5 gallons per crop) filled with the system nutrient solution to determine if the solution volume is what’s causing the deficiency.

Severe Basill Deficiency.jpg

Above is an image of the most affected individual with the Mg deficiency. This image is a good representation of how the Mg deficiency develops in basil. It doesn’t appear to be getting any worse at this point. The crop pictured will also be the 5 gal bucket transplant.

Up to Date Media Bed Crop.jpg

Above is an image of the media bed and all the crops to date. Below are the kale seedlings.

Up to Date Kale in Float Bed.jpg

The outdoor seedlings look fantastic and are growing very well thus far after almost a week in the outdoor bed. Sludge and excess drain water from the system has been used to provide nutrients and communities of microorganisms to the outdoor grow beds. The crops have already showed significant growth since being transplanted on Friday (10/28).

Up to Date Outdoor Raised Bed Crops.jpg



Topics: Aquaponics, Aqueduct Development, Plants, Water