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Alkalinity – How much is enough?

It has been a big week for Alkalinity questions. We have talked about the CO2 issues that arise from CO2, low pH and alkalinity and we will talk more about this topic in regards to bio filtration capacity. For now I wanted to cover some simple guidelines for adding a buffer solution to manage those pH swings and dramas that come from neglecting the carbon requirements of our aquaponic or aquaculture systems.  Well here is the table. Really, we are going to go simple and straight forward to the point with the following table that you can use to work out how much buffer you will need per kg of feed fed to the fish. This table is for use with integrated systems where denitrification is a process of the plants. Sorry getting off track, here is the table. Double the ammounts for straight up aquaculture. Buffer g/kg of Feed Notes… Calcium Carbonate (Calcite) 70 grams Very slow to dissolve 1Calcium Hydroxide (Builders lime) 55 grams Dissolve prior to addition is caustic Calcium Oxide (Slaked lime) 40 grams Highly reactive handle with caution 2Potassium Hydroxide 80 grams Dissolve prior to addition Dolomite 70 grams Very slow to react Magnesium Carbonate 60 grams Very slow to react, may be difficult to get Magnesium Hydroxide 45 grams Very slow to react   At least a little detail: 1 and 2 Are the most commonly used in aquaponic systems are both readily available, though the potassium hydroxide may be a little more difficult and expensive to get.  Often hydroponic stores will stock it. Best so avoid sodium bases (or buffers) in planted system, even though the sodium will be taken up to a minor extent. They are the quickest to dissolve and completely dissolve fast in comparison to other buffers. The magnesium supplements are the slowest to dissolve and not much of it will dissolve.  Best to use magnesium sparingly for deficiencies in the plants and add a small amount of general hardness to the water. The calcium buffers are the better option for leafy green production though not to be used within two weeks of potassium buffers or precipitation will occur.  Basically, a waste of time.  Alternate between the two every odd week should you have a mix of plants, some fruiting some greens. Alkalinity above 70 grams and no more than 120 mg/L will see you right.  Keep it at the low end of the scale for low pH systems (<6.5).  With those low pH systems it may be best to run at around 50mg/L and allow for reduced biofiltration (eg add an extra bed). We will talk more about the low pH systems in later posts. Handle all hydroxides and oxides with care around water as it is very caustic (burns or exothermic). Never add hydroxide or other buffers when ammonia levels are over 2 on your test kits, it will get nasty for the fish. You can use more or less from that table depending on your test results. It is a flexible and dynamic system so go with the flow and keep an eye on the CO2 and keep it below at least 20 mg/L.  You may not notice the effect high CO2 has on your fish but you will tip food in and gain very little conversion. Generally buffers are best applied continuously to prevent pH fluctuations. Simple enough to do with a premix in a soft drink bottle hanging over the sump and some 4mm irrigation tube and a valve sticking out of the bottom. I recommend you test your alkalinity at least once a week (7 cents) until you are satisfied your dosing system is set accurately enough and adjust with your feed as it increases….

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