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It seems to me that pH moderation is constantly coming up as a  subject in just about every discussion.  Because it's such an important detail of all aquaponic systems, I thought that I'd start a discussion to let folks chime in on what has worked and what hasn't worked as far as controlling system pH.  I know that some of you will say that the subject has already been talked to death in other discussions, but I think that this will be a great resource for folks who don't have the time to wade through the other discussions to glean details.  So experienced aquapons (that means you- Aleece, Murray, Rupert, etc.), and anyone else with questions or advice:  let's make life easy for the newbies and share some pH knowledge.

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Well, I guess I'll kick this off. pH is the measurement of acidity of your system. pH measures the numbers of H+ ions in your solution. There are a number of ways to measure pH (the kits you buy at pet/aquarium shops work great), but the easiest and best way I've found is using a handheld pH probe/meter (Testr3 meters). Why is pH important? Well, if your pH values are not within certain parameters you can have serious problems with nutient availability/uptake (especially this!), nitrification processes, and plant and fish health. I've found that the ideal range for my system is between 6.5 and 7.0 although I'm consistently fighting a dropping pH. Most healthy systems will struggle with pH lowering over time due to a number of processes, including:
nitrification - for every ammonia/ammonium ion converted to nitrate, two H+ ions are released into your solution
release of organic acids- in my system I get humic and tannic acid accumulation from peat leachates and decaying plant matter.
Some folks buffer their systems with some kind of carbonate, or use a carbonate based mineral to moderate pH (i.e. lime). I use agricultural lime in my system although there are other types of lime and/or carbonates that can be used.
Thanks for starting this, Nate. Here are my 2-cents as to how to adjust pH that I've gathered over time...

Increasing pH
sodium bicarbonate (baking soda)
limestone
calcium carbonate (egg shells, snail shells, sea shells)
pH Up for aquariums

Decreasing pH
vinegar
pure citric acid (lemon juice, lime juice)
iron sulfate fertilizer (available at your local mega-mart store)
pine straw, deadwood, peat moss
pH Down for aquariums

I'd be interested in knowing what people have used and what they think works best. Me, I've mainly used hydroponic pH Down which I know is a bit of a no-no, but I have a gallon jug of it that will probably last me the rest of my life...and the fish don't seem to mind. But I don't recommend it based on what I've read (do as say, not as i do...)
Increasing pH

Potassium Bicarbonate (strong buffer - added benefit of potassium for fruiting/flowering)
Slaked/Hydrated Lime - Calcium Hydroxide (very strong buffer)
Dolomite Lime - Calcium Magnesium Carbonate (moderate buffer - increases general hardness)
Calcium Carbonate (egg shells, snail shells, sea shells - moderate self regulating buffer)
Limstone - can hold pH from 7.8 - 9.0 - (Moderate - High, persistant buffer - increases carbonate hardness)

Decreasing pH

Vinegar (moderate acid buffer)
Pure Citric Acid (lemon juice, lime juice - moderate acid buffer)
Hydrochloric Acid (Muriatic Acid, Pool Acid - very strong acid buffer)
Sulphuric Acid (strong acid buffer)
Coco Coir media - often 5.6 - 6.4
Maidenwell media - Diatomite - can be 5.2 - 5.8
Before struggling with trying to change a system's pH I believe it is very important to sort out why the system is the pH it is.

Let me elaborate. Many new systems will start with a high pH simply due to hard source water. In such a situation, the pH can be brought down some with acid and once the system cycles the trend will generally continue downward.

However, I've had people come to me with issues of their pH being too high and they add acid which brings the pH down for several hours but by the next day the pH is back up where it was before. The problem here is often that there is a really powerful buffer in the system. Like perhaps they used limestone as their gravel. You are not likely to satisfactorily lower the pH of a system using limestone as media. I have sea shells as about 40% of the media in my big system, the pH of my system is definitely buffered and adding acid won't bring my pH down for long. The shells buffer to 7.6 and I've rarely gotten the pH down below 7.4 (only during rainy season when it is very warm and the fish load is heavy and the fish are eating lots along with me supplementing extra Iron.)

If pH is high due to concrete tanks, I'm not sure what the best answer is. I know some people with concrete tanks and they have great systems. I don't know what was done to the concrete to make it not affect the pH too much and I know many other people that have issues with high pH and concrete portions of systems.

Anyway, I don't advise using a strong buffer as your grow bed media. It might be handy to have some limestone and/or shells on hand to add to the system in small amounts to keep pH from dropping too low but when it fills your grow beds, there is no artificially lowering the pH until all the buffer dissolves away.

I also have really hard well water. As I noted in my Blog post pH and Tap Water You need to air out the water before trusting the pH reading of tap water. Even with my high pH well water (once aired out it will be as high as 8 to 8.2 depending on season) if I set up a system with pH inert media, once the system cycles up, the pH will drop. Topping up will affect the pH up but I would still need some shell grit on hand to keep the pH from dropping too low over time or when rain water is topping up with acidic water for me.

As to what pH is best, well I can't really say since I've been struggling with my choice of shells, but I would say to keep the pH enough above 6 so that it remains readable to the test kit. Problem with the aquarium pH test kit is that if you are running a pH near 6, you might not realize when it actually drops below 6 and once it's below 6 it could be anywhere. Eventually, the pH gets so low that the nitrogen cycle breaks down and the system crashes with Ammonia spiking and other problems. So to be safe I would say a pH between 6.5 and 7 should be perfect.

But take heart, there are still plants that will grow in a system with a pH above 7. I've got lots of tomatoes and peppers as well as basil, papaya, rosemary, bananas, bamboo, Artichoke, and in cool weather lettuce, broccoli, kohlrabi, cabbage, swiss chard, and many more do grow.
Just to add to the pH discussion.
AP systems work best with the pH just below 7.
A mature system will always slowly drift down in pH and will need to be buffered up by using Hydrated Lime or similar.
I usually let mine drift down to about 6.2 before taking action to buffer. This cycle is usually about once a fortnight but will vary according to season and the size of your AP system.
I don't know exactly how much of any particular acid you should be adding. I have used pool acid to adjust pH before and I would suggest reading the label to sort out how much to add for a particular volume of water. Have you been fishlessly cycling? If so, once the system is cycled, the pH usually drops on it's own.

(I don't know that pool acid is really such a great thing or really safe to use but in very limited amounts it does not seem to have hurt the tilapia I had in a system that I adjusted the top up water with it.)
To add in my thoughts, I use pH down by General Hydroponics mainly because I have a gallon jug of it, which is pretty much a lifetime supply. Again, like TCLynx, perhaps not the best way to do it (I'd put my money on Murray's method) but in small doses it is very effective at moving pH and doesn''t seem to bother the fish. I caution you that whatever you do, do it slowly. the rule of thumb I was taught was about 1 - 2 tenths of a point per day, otherwise the fish can't take the sudden plunge in pH. I've lost a few this way.
I'd probably add that 8th of a tsp to say a 5 gallon bucket of your system water and then see what the pH comes out to for that. How much water is in your complete system? Hard to tell how much to add if we don't know what we are adding it to.

If your system isn't really big, adding a tsp of this acid here and there could actually be hindering your cycling cause if you are causing sudden extreme pH drops in parts of your system that then comes back up and the acid dilutes, you could be killing off bacteria as it is trying to get established.

I would think that diluting the acid in some system water and then slowly adding that to the system might be a better way to go about things. Even if you don't have fish in the system yet, sudden extreme pH changes or pH bouncing can still hurt things. Unless something in your system (other than just the water) is buffering the pH up, cycling will naturally bring the pH down usually about the time you get past the big nitrite spike. How far along are you in the fishless cycling process?

Pamela Urbas said:
I am doing fishless cycling. Due to the extremely high ph of the water, the slightest hint of ammonia does a number on any fish.
I've started adding an acid blend used in brewing. Unfortunately it has no instructions for how much to add for a ph level but an 8th of a tsp does a number on a gallon of water (dropping it from high 8s to about 3) so I've been adding a tsp here and there and checking the levels.
so how much change is a teaspoon of the stuff causing. Like if you test pH on the system then dissolve some of the acid in a 5 gallon bucket of the system water (so you can take your sample reading) and then pour the 5 gallon bucket that has been adjusted back into the system, wait say through a pump cycle or two then test the pH again. If that amount of acid managed to move the system pH say .2 then test again the next morning to see if the adjustment actually held (if the adjustment doesn't hold then it might indicate there is something in your system buffering it back up.)

If that adjustment of say .2 worked you know how much to use each day to move the system pH down till you get to around 7. I would stop at 7 since once the system cycles the pH is going to try to drop even lower and you will need to buffer to keep it up.
I`ve just started up too. My pH is up and has been up for a week now, at 8.5. My amonia is 0. I`ve not added anything yet. Except for a brief moment we had some minnows from our pond, i just got too excited to wait any longer. So with the fish came a little pond water too. They all went missing overnight. My media is river rock and i have about 50ish gallons of water. We have some humonia standing by ( i had some very willing volonteers). Do i get the amonia up or play with the pH first?
Provided there is no limestone, marble hiding in your media. I'd just start fishless cycling (look up instructions in my blog). I found that even with my hard water, the pH dropped about the time the nitrite spike started dropping. If the pH isn't down by the time you are done fishless cycling, then it might be time to research ways to moderate it. It might be as simple as needing to adjust the pH of your top up water before topping up.
Ahh PH the scourge of aquaponics. My well water is naturally 7.4 and I dropped it to 6.8 when I was running Hydroponically by adding 1/4 cup vinegar to 30 gallons of water. So I set up for aquaponics with a 600 gallon pool with heavy aeriation, no limestone or anything in the system, all neutral. I do the math - 1 1/4 cups vinegar should do it.

It does for about 2 weeks then things went wacky... my PH shot up to 8.0, I added vinegar and nothing changed, added more... then more eventually over a week I added about 2 gallons of vinegar and saw no PH change what so ever.

Time for the big guns and got some Muratic Acid. Added 1/4 cup waited 4 hours and tested - 6.9 Success.

Until the next day -- it is back to 7.6. What is going on?

I added another 1/4 cup wait four hours and test - 7.0.

Test again at 6pm feeding, 7.0

Next morning feeding.. no one eats, huh? pull the cover and I have four floaters. check some bottom and and find two more dead. crud.. test again and it is 6.0 (or lower).

That evening I check... it's back to 7.4 and fish are eating a little. But I did nothing?

Next day, 7.6 and has held there.

I'm not touching it now until Sunday, then will test again and slowly try to adjust again.

I have no idea what happened or why, But I'll try again.

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