Our advice to lower pH in aquaponics
High pH is a common concern among beginning and intermediate aquaponic growers, who want a quick fix to bring it down.
My philosophy on lowering the pH in a system with high pH is simple.
If you’re thinking about using a chemical or acid to lower your pH, don’t. You’re just going to make your life more costly and complicated than it needs to be.
High system pH is almost invariably caused by carbonate buildup in the system (when it’s not carbonates in the form of limestone grow bed media, concrete in the fish tank, etc.).
When it is carbonate buildup in the system, your pH is being “buffered,” or maintained artificially high by the carbonate cycle of your water.
Carbonates can be a complicated subject, so I’m not going too complicated here. The thing to remember is that carbonates have the ability to dissolve into solution and precipitate out of solution.
The way that they do this maintains pH in a very specific high range until one day, you’re adding an acid like you usually do, and your pH crashes. Bottoms out. Drops like a rock.
When this happens this quickly in high pH systems, the first thing that happens is nitrification stops- which we all know is a very bad thing.
How to Safely Lower pH
So what are you supposed to do?
If your water isn’t too hard (high carbonates), you can eventually get your nitrification to the point where your acidification from nitrification outstrips the carbonates entering your system, leading to a naturally low pH.
When done correctly and slowly, the nitrifying bacteria will adjust and there’s no shock. Once you reach this sweet spot in your system’s health and lifespan, it is a wonderful thing.
At this point you have control of your system pH. Where do you want it? It’s completely up to you.
The best way to keep pH from getting out of control is an RO filter
If your water is harder, or if you simply don’t want to wait, the best thing to do is install an RO filter (Reverse Osmosis). RO filters remove almost everything from the water (including carbonates) leaving you with very pure, carbonate-free water.
RO filtered water is simple stuff.
You can allow nitrification to drive pH down, or you can add simple hydroxides to raise pH (and supplement nutrients) with no long term pH effects.
RO filters can cost a few hundred dollars, but in my experience, many of the folks who spend months struggling with pH, and artificially trying to lower it, reached the point where looking back, they wish they’d just installed an RO filter and avoided the cost and heartache of fighting the fundamental chemistry of their system.
Don’t forget about Biological Surface Area
So there it is. You have two solutions- either be patient or drop some money on an RO filter.
If you do choose to wait, I do recommend using a growing technique that contributes lots of Biological Surface Area to the system.
This increases the potential amount of nitrifying biomass in your system as well as the resulting efficiency in nitrification. This can lead to faster nitrification based carbonate consumption.
We use ZipGrow Towers for this purpose because our media has massive Specific Surface Area (SSA) and really helps get nitrification at peak efficiency.
I hope this is helpful for you and really commiserate with those of you who are struggling with this issue.
Controlling your system pH can be an incredibly frustrating task, but if you follow my advice, you can take the first steps towards taking control of your system and how it operates. Leave a comment if you need help!
for the night! I need help in setting up ph and kh. I was already tortured. as well as I have increased ammonium compounds. help with advice
So are we talking about RO filtering the water out of the fish tanks and putting it directly back into the water? Do you stop once you’re kh gets to a certain point? Once you remove the entire buffer won’t there be dramatic pH swings? I have a pH of 9 in my 700 gallon system. I want to lower without crashing and I need to get plants going sooner rather than later.
Lyndon,
Filtering your system water through an RO filter is not worth it. Filtering your source water and then using it to top off is the best way to manage it. You can also burn through your buffer with acid additions, but this can be rather tricky and hard on your nitrifiers. I would recommend being patient, topping off with lower pH/carbonate water and watching pH carefully over acid additions.
This is solid advice, but advanced aquaponic growers routinely add phosphoric acid and potassium hydroxide to their systems NOT to balance PH, but to add potassium to their system. Potassium supplementation in systems is a huge pain, as plants need a LOT of it and most plant nutrient issues you’ll encounter in aquaponics is caused by low K+. A variety of acids help correct the effects of adding KOH- to your system. Unless you’re backing your additions with well-thought out calculations, i’d strongly reccomend you heed the advice here as you can get yourself in bad water real quick if you’re not a measure twice cut once type of person. Feel free to chime in if you agree/disagree 🙂
Can we use sodium based water softener to remove hardness in source water ?! My source water has total hardness of 625ppm … RO is a costly investment …
Nisar,
Avoid putting sodium-softened water in your system—it’ll likely kill your plants.
My issue is I’ve been running my pump and grow bed for 2 months without fish…now my fish are here and ph is 8.8…was 6.9 2 weeks ago. Have only topped off tank with ro water 4 a month
Can’t add fish to tank to 7.6…is there a way to close the gap for the new fish…bill
Hi Bill,
Phosphoric acid is pretty safe for aquaponics, just remember to make changes slowly to avoid harming the fish. Also, use grow media with a high biological surface area to aid the nitrification process, which will help stabilize pH at a lower level.
Hi, I have a concrete pond about 700 gallons + 150 in filtration system. The concrete portion is newly resurfaced to fix old leaks and I would think that using RO water would leach out or etch the concrete since there is no hardness or enough dissolved chemicals. Pools that do not have high alkalinity and ph above 7 will strip cement plaster. Is this not true for ponds?
Reverse Osmosis is NOT filtration, it is a membrane permeate process, and if done correctly, will have a waste (or “concentrate”) stream of 15-25% of your feed flow, depending on your feed TDS. Prefiltration is recommended. Hollow fiber membrane technology is not.
Hello.. Does the stocking density of fish affect the PH as well ?
I am curently running 1 FT ( 1000Ltrs) 1 sump tank ( 1000Ltrs) two media beds and 1 DWC bed. At the moment my system has been running for approx. 2.5months and my PH has been constant in the range of 7.2- 7.8 I have stsrted growing plants but soon relaized leaves were turning yellow, so I added Iron chelate and I am seeing improvements to the plants. However, growth is slow due to PH being high.
I have 15 tilapia fish in total but they are max. 10-15cm long. My question is should I add more fish to combat the alkalinity of the water to drop the PH ?
thanks.
Hi Julz,
The stocking density does matter but you’ll want to pay more attention to the amount of protein you put in the system, which is dependent on stocking density. The more fish you have, the more you can feed, which will drive your pH down.
In my system water is hard. And my system pH is at 8.2. if I increase biological surface area like media beds is it benificial for controlling pH.
I just finished building my first aquaponics system which consists of a 1000 litre IBC fish tank, a 1000 litre IBC filtration unit filled with Dacron, 2 x 2000 litre deep water beds and 1 x 2000 litre gravel bed all made of fibreglass.
I used a 5mm -10mm river gravel for my gravel bed which I randomly tested with vinegar for carbonates but once I got the system circulating with water I tested the pH and it was 8.4. The level comes down fast when I add muriatic acid but by testing time the next day its back to 8.4. Actually 8.2 today. All up I’ve added about about 1 litre of muriatic acid to the system so I’m not sure what to do next? More acid….
The good news is that after about 7 days from when I started the fishes cycling process (using ammonia sulphate) I’m starting to see an increase in the Nitrates (0.25) and 5ppm nitrate.
My big issue is how to deal with the pH as I think there might be carbonatius rocks in my gravel bed that for some reason never reacted with the vinegar.
I am really worried about the ph levels as my well water is super hard. I used electric descaler to soften the water but ph is level is not balanced as it is more than 7