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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:
Cheers
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
