I will caution that too much lowering pH with acid is probably going to cause you other problems and you need to look into the reasons you are needing to low pH all the time. Doing it once before start up may be ok but I would only lower it to the mid 7 range since the bacteria will lower it more later. For doing that I usually use Muratic acid (hydrochloric acid) used for doing acid staining of concrete. I have also used pool acid for this purpose before but I wouldn't recommend using it regularly. There are also some pH down products that may work but they usually cost more. Vinegar might work in a pinch but I would avoid citric acid since it has antibacterial properties.
If your media is full of calcium carbonate, don't bother trying to lower the pH with acid, it will only cause pH bouncing and you will have too much calcium in your water and you won't be able to provide enough potassium to your plants.
To buffer (raise pH or keep it from falling too much) the normal recommendations are to alternate a potassium buffer with a calcium buffer.
Calcium buffers include
calcium carbonate (this is slow acting and includes lime, limestone, egg shells, shells, coral, and shell grit, etc) Generally I like to put this in a mesh bag or stocking so that if the pH gets too high I can remove it.
calcium hydroxide (hydrated lime or brickies lime) This one is strong and fast so be careful you don't use too much since moving the pH too much in one day can hurt your bacteria, fish and plants.
Potassium bicarbonate (find it from wine making suppliers) will buffer pH up fairly quickly and provide potassium at the same time, can also be made into a spray to use on plants to combat certain types of fungus and mildew.
Potassium hydroxide KOH (this is like old fashion lye) be careful with this stuff. It is really fast acting and strong. It can also burn you. The big commercial operations often alternate this and the calcium hydroxide and they usually use a special separate tank to add these things slowly over time into the system so that it doesn't cause the pH to change too rapidly.
One can make their own potassium pH buffer using wood ashes (soaking and leaching wood ashes is how they used to make old fashion lye) However I don't know the details about how much you are supposed to use and how long you soak and all that so be careful in your experiments.
You just keep pouring your water (dont use tap water, rain water or maybe these days distilled) back through your barrel (preferably with a hole near the bottom, one that you can 'cork' up with something) that you've prepared...some rocks on the bottom and straw over the rocks then ashes over top the straw. Collect the lye solution in a pan (non-aluminum) and keep pouring it back to filter through the ash, (straw and rocks are just filter media I believe).
My grandma used to make the stuff in this manner for her soap. Now, she did have a couple ways of testing the strength of the lye, and both of them involve chickens ...Pluck a father off a chicken and dip it in the lye. If it dissolves it's strong enough for soap. Very scientific and all. If you dont have access to chicken feathers, you can try to float an egg in your lye solution. If it sinks - too weak keep pouring through the barrel, if it floats a top breaking the surface - too strong, dilute with water. If it floats just under the surface of the water - your Goldilocks...
I'd probably just let the solution dry in the sun, evaporate the water till just KOH crystals are left. You could then measure out and dissolve in water (probably distilled) to a desired calculated strength (or an approximation thereof at least).
Though it's probably best to just order the Potassium Bi-carbonate if that option is available. Lye is pretty hardcore caustic stuff.
Cool Vlad, thanks for the description about how to leach ashes to make lye. These old fashion tricks can often be handy to know.
Of course for use in aquaponics you probably wouldn't need to leach them to the point of lye and you could probably use the weaker solution for slower pH adjustment without as much danger from the extremely caustic full strength lye.
But the point about the Potassium Bi-carbonate is good too.
And the potassium bicarbonate can also be used as a fungicide spray.
Yeah, it's hard to beat the triple whammy of protection against downy mildew etc... (when used as a foliar spray of course), getting potassium to your plants and buffering your pH all from the same purchase...safer to use too.
Is Sodium TetraBoratePentaHydrate ok to use as a buffer?
Jim, I've never heard of it. I would usually say avoid the aquarium stuff as it will cost more and could also be adding things we don't want (like we want to keep the sodium to a minimum when we can.)
In general to buffer pH up we have a choice of strong or weak.
Strong would be alternating Calcium hydroxide with potassium hydroxide but these are really strong so you have to be very careful.
Weak would be alternating calcium carbonate and potassium bicarbonate. These are weak so you have to be very patient and don't go adding more a few hours later if it hasn't changed the pH cause if you add too much and don't have a way to take it out it can be a problem too.
If you have hard water, that will probably substitute for using the calcium buffer so you will probably only need to use the potassium buffer and you might need some source of less hard water to keep from getting too much calcium carbonate in your system.
Thanx, I'll look those up.
SodiumTetraBoratePentaHydrate is Borax... water softner.
Stay away from DiHydrogen Monoxide though... that stuff can be deadly :)
I don't think much Borax would be safe for the fish.
Borax dissassociates into Boric acid... and/or "borates"...
Although boron is an essential micro-nutrient for healthy growth of plants, in can be harmful to boron-sensitive plants quantities. Plants and tees can easily be exposed by root absorption to toxic levels of boron in the form of water
Animal test show signs of effects on reproduction....
I wouldn't use it.... if you need to bring pH down... use Hydrochloric acid...
I would never use Borax as a pH buffer- I do use it in my system as a nutrient supplement but very seldom. To use it to move pH significantly would require more than you would ever want to add to your system. i.e. what Rupert said. Micronutrients are micronutrients. High exposure almost always leads to a toxicity or nutrient lockout problems. . .
Also, I'll add that if you ever use wood ashes to make lye you want hardwood ash. Softwoods like pine don't make good lye. [My wife makes soap for us. Bear fat soap- best soap on the planet by the way. :) ]
I have used muriatic acid one cap per hour in 400 gallons testing after it mixes for 30 minutes minimum and plants like the acid low doses wont hurt tilapia but coulds hurt others but you should find the reason its high