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How to handle Potasium/ Calcium supplimentation when Ph never goes below 7.4

Hello Everyone,

This is my 7th year running my Aquaponics system. This is the first time I have run into a problem where the Ph has been hovering between 7.3 - 7.6 for the past 4 weeks. I built a hybrid system that consists  of a 300 gal fish tank. The fish tank has 80 Tilapia fingerlings. The system also has 3 Deep Water Culture  bins where I grow lettuce and 2 grow beds with hydroton growing Bok Choy, Peppers, Cuccumbers, Tomato's, Basil. The entire system is in a hoop house that gets shut down and cleaned out roots and all in the winter because its just too cold and not cost effective to run it year round in New York. The only thing I am doing differently is that this season I have planted alot more plants than I have in previous years because of the amount of Tilapia I am raising. I start all my plants from seeds indoors using Rockwool. There is alot more Rockwool in the system. The first thing I checked for is fish deaths. The fish are fine. The Tomato plants and Cuccumber plants have shown signs of Chlorosis. I tried using a foliar spray of Potasium Carbonate and burned my plants. Luckly I only lost a few of them. I have supplemented Chealated Iron first to see if that would solve the problem. No luck. Thats when I tried foliar spray of Potasium Carbonate.

What have you done to suppliment Potasium , Calcium with such a Ph? Why do you think the Ph is not falling like it should? Water temp is in the low 70's. Its heated. Ammonia and Nitrite is 0 ppm.

Thanks for your time and comments.

Rich K

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It's been a while since you posted. I'm surprised no one has replied. If your ph is 7.4 it's too high for good plant growth as you probably know. I have a similar problem and similar system. I use muratic acid to lower ph in low doses. You can lower the ph to say 6 and dose with potassium hydroxide and lime to raise it a little while supplying your nutrients or you could use potassium sulphate and calcium nitrate to just add nutrients. Skip the lemon and lime juice for lowering ph. Muratic acid is cheaper and faster.

Hi Richard,

You can try adding a product like Aquaponic Elements into your system.  Among other nutrients, it contains potassium, calcium, and iron. You just add it to your sump tank and it dissolves in over time.  Dropping your pH should help a lot as well.  I use phosphoric acid, which is in products like AquaDown.  It's probably taking a while for your pH to drop if you have a lot of buffer in it, like calcium for example.

What are your nitrate levels?

Hello Rob,

My Nitrate levels always run very high. No matter what I do. I use the API test kit. The Ph problem seems to have fixed itself. 

I have been adding M1 Biological but I dont know if it is making any difference. Its September and the garden is winding down. I close the greenhouse in December because it is too expensive to heat the building and water on Long Island. I start up the greenhouse again in March.

Thanks for your input!

Hey Richard. As far as the nitrates go, yeah, I've been there myself. I take it they are pretty much off the charts and testing at a beautiful ruby-red color? At that point, you can do partial water changes until you get your nitrates down, but I find that to be much too much work. I prefer to transfer the fish to a separate container filled with system water and a good amount of supplemental air while I drain, replace, and treat the remaining water back to where it should be. Once I've got matching pH levels between the fish container and the rest of the system, I go ahead and add them back in.

If you're using well water and if you don't have one already, I would go ahead and invest $8 in the API KH test kit for freshwater: https://www.amazon.com/API-TEST-Freshwater-Aquarium-Water/dp/B003SN...\

The pH issues you had earlier might be a result of high carbonate hardness (KH), which acts as a pH buffer. Your pH won't lower until whatever acid you've been adding neutralizes all the carbonates in your water. This can take a good bit of acid if your source water is high in carbonates (mine tests at 18dKH), so use the cheap muriatic acid you find at your local hardware store and save your $20 bottle of phosphoric acid for minute adjustments.

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