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Maintaining a low pH in blackwater aquariums

Ben Rhau

Apisto Club
Messages
331
Location
San Francisco
Hi all,

I'm in the midst of several blackwater tank builds, and thought I'd share some of my findings with you. Though they aren't being designed for cichlids, the same principles apply to the water chemistry.

My target pH is 4.5 and my target conductivity is below 50 us/cm. Starting with RODI, the common methods of lowering the pH are strong acids or peat. For sustainability reasons, I don't want to use peat. I briefly investigated using shredded redwood bark as a peat alternative. I may post more about my negative result in the future, but the short answer is that steeping with redwood bark didn't lower the pH of RODI water. (It did lower the pH of alkaline tap water, however).

So I pivoted to strong acids. I know this is a bit polarizing, as some people have no problem doing it, and others find it too hazardous to advocate for most aquarists. I think it's fine to do given some precautions (listed below). Credit to @Joshaeus, an occasional visitor here, and from whose posts on other forums I learned these techniques. My observations largely confirm his:

1. It takes regular effort to keep the pH below 5.

Conventional wisdom says that very soft water tanks are prone to pH swings. That's not what I've observed over the last 2 months. The resting pH of my RODI water is consistently about 5.5. I can lower the pH of the RODI water to 4.5 by adding 2 drops of 1M sulfuric acid (or 1M phosphoric acid) per gallon. However, the pH always drifts back up due to the carbonate-carbonic acid equilibrium. i.e., The CO2 in the air is always adding carbonate buffering to the water.

To overcome this a bit, I first add an excess of acid (for example, 1.4 ml of 1M sulfuric acid per 10 gallons of aquarium volume) to neutralize the carbonate and move the equilibrium lower. Once this is achieved, I add 25-50% of the volume of acid that was required in the first step with each weekly water change to hold the pH between 4.5 and 4.9. The exact amounts are determined empirically and vary slightly by tank.

Note: I alternate between using sulfuric acid and phosphoric acid, since the residual ions are both macronutrients for my plants. Hopefully, the plant consumption of the ions will lower my conductivity a bit and benefit the plants, which otherwise don't get any added fertilizer.

2. If you start with RODI, you can still have low conductivity.

If you started with hard water, it would take a lot of acid to neutralize the KH, and that adds a lot of ions. So this would not be a good solution for lowering the pH of hard water. However, since I'm starting with RODI, it doesn't take much acid. Even with the initial high dose of acid, I can keep the TDS to about 30 (EC of 50). Adding less acid with subsequent water changes allows me to slightly lower it from there. Between the acid and the botanicals, I can't get it much lower than an EC of 40.

Note that I followed these precautions:
  • Use a dilute acid. I'm using concentrations of 10% and below, although much more concentrated acids are cheaper and available. Those are hazardous to work with at home, however, where I don't have a fume hood.
  • Be careful anyway. I always wear nitrile gloves, use a secondary container for my bottle of acid, and generally try not to multitask or have any distractions while I'm handling the acid.
  • Only add acid to water outside the aquarium. If there are no animals in the aquarium, I'm OK adding acid directly. Otherwise, I only add a little at a time over small water changes, always mixing outside the tank.

Cheers
 

Frank Hättich

Well-Known Member
5 Year Member
Messages
308
Location
Germany
For some years now I'm using 4% hydrochloric acid and more or less had the same experiences as you. I can add that in my tanks the pH eventually becomes stable at the desired value (in my case 5.0-5.5) usually after 3-6 months.
 

anewbie

Active Member
Messages
476
Is it necessary to use acid or will pure rodi water with peat be sufficient. Is there a benefit to using acid over peat?
-
Next year i will setup a 4ftx4ft cube low ph tank so just trying to collect information. I had planned to stuff peat behind the matten filter and use 10:1 ro water (the 1 tap was to get some minerals as our tap is kh 3 gh 7); but when i read threads like this i keep thinking i should use acid instead of peat for stability.
 

MacZ

Well-Known Member
Messages
1,131
Location
Germany
Is it necessary to use acid or will pure rodi water with peat be sufficient. Is there a benefit to using acid over peat?
Peat + RO is sufficient. Acid has the benefit of lowering pH down further down than peat and in contrast to peat it is also more sustainable to source and easier to store.

I think it should be noted that to keep the pH actually stable it still takes humic substances. Lowered just with acid there is no stability as with no KH something has to take over the buffering. So acid alone is no replacement for peat, leaf litter and/or botanicals.

I had planned to stuff peat behind the matten filter
In my opinion a guarantee for foulness.

and use 10:1 ro water (the 1 tap was to get some minerals as our tap is kh 3 gh 7);
In a 1:10 ratio and those tap-readings you can ommit the tap completely, boils down to the same.
 

MacZ

Well-Known Member
Messages
1,131
Location
Germany
Why? If the peat is in a filter bag why would it result foulness ?
You wrote "stuff". I understood that as a densely packed, big amount of peat.
Flow behind a HMF is not very strong, so anaerobic zones in the peat are possible.
 

Ben Rhau

Apisto Club
Messages
331
Location
San Francisco
Is it necessary to use acid or will pure rodi water with peat be sufficient. Is there a benefit to using acid over peat?
-
Next year i will setup a 4ftx4ft cube low ph tank so just trying to collect information. I had planned to stuff peat behind the matten filter and use 10:1 ro water (the 1 tap was to get some minerals as our tap is kh 3 gh 7); but when i read threads like this i keep thinking i should use acid instead of peat for stability.
Most keepers seem to use peat, which is certainly sufficient but unsustainable.

It's possible to use a combination. I've seen lots of folks use peat in the substrate as a partial stabilizing factor and also add oak extract (which is basically phosphoric acid). Problems with peat in the substrate:
  1. Eventually, the ion exchange sites will exhaust, so it will lose its acidifying properties over time. Then you'll need to swap it out, which is a messy endeavor.
  2. You don't have control over the exact parameters of the water unless you treat it outside the aquarium. This requires more space, equipment, and time. That approach also solves for #1. I used to use a peat cannon, but switched to acid.
I think it should be noted that to keep the pH actually stable it still takes humic substances. Lowered just with acid there is no stability as with no KH something has to take over the buffering. So acid alone is no replacement for peat, leaf litter and/or botanicals.
That’s correct. My tank has lots of structural leaf litter, driftwood, seed pods, and alder cone extract. I do indeed observe buffering. Adding alder cone extract to RODI in my routine reduces its pH from 5.5 to 4.8. However, the tanks with freshly added alder cone extract increase in pH more rapidly than theh tanks where I water change with RODI alone (from 4.5 to 4.8).

Cheers
 

Ben Rhau

Apisto Club
Messages
331
Location
San Francisco
Which is a rather unspectacular small change.
By itself yes, but small changes are the goal. If it continues to rise to above 5.5 before the next water change, I need to reduce by a full pH unit. I can do two smaller changes, but it’s easier if I can keep it to a tight range. That’s why I care about the speed of the pH drift.
 

MacZ

Well-Known Member
Messages
1,131
Location
Germany
By itself yes, but small changes are the goal. If it continues to rise to above 5.5 before the next water change, I need to reduce by a full pH unit. I can do two smaller changes, but it’s easier if I can keep it to a tight range. That’s why I care about the speed of the pH drift.
I see. The temporal factor was missing. Now this makes more sense.
 

Apistoguy52

New Member
Messages
16
+1 for hcl/muriatic. The precision coupled with the ability to easily manipulate water for “seasonal” changes makes it a winner in my book
 

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