Using the hydrochloric acid to lower alkalinty

Aprilbeingbasic

Member
I will preface all of this by saying YOU ARE RESPONSIBLE FOR YOUR FISHES AND YOUR OWN SAFETY. It is not my fault if you cannot be logical and handle things with care/test accurately before adding things to your tank. But I do however want to discuss this, seemingly, hush hush idea of using hydrochloric acids in tanks to lower alkilinity.

3 weeks ago I tested my tap water to see it had gone way up to pH 8.6 with a GH of 180ppm and KH 120ppm. Even though I know alot of the times it is better to just leave pH instead of messing with it my fish were showing red gills and I was starting to get low ammonia readings (which is much more toxic in high pH) likely because my beneficial bacteria work slower in pH out of their suitable range (6.5-8.0). So I decided I needed to lower the alkilinity. Now, Im a scientist and I actually work with acid nearly everyday, the most obvious answer to me was to lower my water change water in a bucket with dilute pool acid (hydrochloric acid or HCl) before adding it to the tank. The reaction between carbonate and HCl is-

H+ + HCO3- --> H2CO3 --> CO2 + H2O

The left over products are just water and CO2 that gets gassed off. So completely safe for fish. I always put my water change tap water in a bucket first anyway to condition it and let it get to room temperature, so I knew the reaction between the carbonates and acid would be completely finished before the water ever got to my tank.

I went looking for information on this and found only early 2000s forum questions on it, with a bunch of people thinking looking at acid burns you and then a few other people saying koi farmers had been using it for ages. However all the domains were too old so I never found a direct source of information regarding the use of it in fish care.

I had enough hints at information this was safe, aswell as enough chemistry knowledge to feel it was and went ahead and bought some pool acid from the hardware store.
most pool acid is 32% concentration. I got an airtight jar with a PLASTIC lid (HCl corrodes metal, not glass or plastic) and diluted the pool acid 1:2 (down close to ~10%). When filling up my bucket of tap water I added 1.5ml of the diluted acid and added an airstone to help the CO2 gas off. After 30 mins I tested the water (you need to wait, not for the reaction as it happens very fast, but to allow the pH to stabilise and buffer). My kH went down ~10ppm and my pH went down ~0.3. I did a 30% water change every 3 days for the next couple of weeks, increasing the acid added by 0.5 ml each time, my tank is now at 7.5pH. Fish are completely fine and have not had any ammonia.

So still confused why this is so frowned upon, but people will spend money on pH down products and just chuck them straight in the tank. Anyways, seen a few posts on here asking about how to bring pH down and thought it was worth saying, as long as you are logical, there is no reason HCl cant do the job. If you have some GH and KH in your tank there will be enough buffer to not let the pH crash.

Not sure if this will make people mad. But worked for me.
 
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ryanr

Moderator
Member
Interesting concept, and certainly makes scientific sense.

Seachem's Acid Buffer uses bisulfate salts, which if my high school chemistry is right, are salts formed from an acid. And when googling, wiki suggests sulfuric acid is often the acid.

Seachem's product works in much the same as your HCl, in that it gasses off the excess KH to CO2. So I'm interested, in addition to dropping your pH, your KH also decreased. Are the two concepts the same?
 
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Aprilbeingbasic

Member
ryanr said:
Interesting concept, and certainly makes scientific sense.

Seachem's Acid Buffer uses bisulfate salts, which if my high school chemistry is right, are salts formed from an acid. And when googling, wiki suggests sulfuric acid is often the acid.

Seachem's product works in much the same as your HCl, in that it gasses off the excess KH to CO2. So I'm interested, in addition to dropping your pH, your KH also decreased. Are the two concepts the same?
KH and pH are very closely linked if your KH is high so is your pH. and generally speaking if you have high KH you have high GH which is also buffering the water. So you do want to bring down your KH if you want the water to stay at a lower pH. HCl doesnt effect GH, which is good because generally GH is good for things in your tank.

But really with all those fish specific products you're just paying for something slightly safer to handle. pool acid is cheap and effective. So thats what I went with.
 

Frank the Fish guy

Member
Yes I do this in my water system. We use HCL to remove excess KH when needed. This allows running low pH aquariums. Standard practice in some fish farms that need it.

The real issue here is that some folks live where the source water comes out with very high pH, say 8.5 due to having high KH. Sadly, ammonia is very toxic at high pH, so sensitive fish do very poorly. Also if you try to use the aquarium pH down stuff, what often happens is that it 'wears out'. I mean the pH comes back to where it was after time.

But if you remove the KH from the water using HCL and aeration, down to reasonable levels you can operate an aquarium at reasonable pH (7.5) where you won't have the problems with the ammonia being so toxic. At 7.5 pH the ammonia concentration that is lethal is 10x greater than at 8.5 pH. So it's a huge difference.
 
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Aprilbeingbasic

Member
Frank the Fish guy said:
Yes I do this in my water system. We use HCL to remove excess KH when needed. This allows running low pH aquariums. Standard practice in some fish farms that need it.

The real issue here is that some folks live where the source water comes out with very high pH, say 8.5 due to having high KH. Sadly, ammonia is very toxic at high pH, so sensitive fish do very poorly. Also if you try to use the aquarium pH down stuff, what often happens is that it 'wears out'. I mean the pH comes back to where it was after time.

But if you remove the KH from the water using HCL and aeration, down to reasonable levels you can operate an aquarium at reasonable pH (7.5) where you won't have the problems with the ammonia being so toxic. At 7.5 pH the ammonia concentration that is lethal is 10x greater than at 8.5 pH. So it's a huge difference.
Yes I completely agree, hence why I did it. because HCl is such a reactive acid it converts the carbonates fully, unlike the organic acids used in ph down products. I totally think it is a better alternative to whatever is being sold in fish stores however whenever I see it mentioned people act like you've suggested adding cyanide to the tank. so scared of it! I mean with any amount of sense you can avoid hurting yourself with pool acid.
 
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ryanr

Moderator
Member
Aprilbeingbasic said:
KH and pH are very closely linked if your KH is high so is your pH.
I'm very familiar with the pH/KH relationship, (and temperature for that matter)
SW Reefs teach you a lot about water chemistry
Some pH down products don't alter KH, which is often the cause of many problems with pH swings.

Aprilbeingbasic said:
But really with all those fish specific products you're just paying for something slightly safer to handle. pool acid is cheap and effective. So thats what I went with.
And in some cases, easier to purchase. Different countries have different rules about buying certain products, especially chemicals, due to their potential to be used for nefarious purposes.
 

JettsPapa

Member
Aprilbeingbasic said:
. . . whenever I see it mentioned people act like you've suggested adding cyanide to the tank. so scared of it! I mean with any amount of sense you can avoid hurting yourself with pool acid.
You hit on the answer there. Many people will react intuitively to some situations or suggestions instead of reasoning through the process. They hear "acid", and all analytical thought stops.
 

Frank the Fish guy

Member
Aprilbeingbasic said:
Now, Im a scientist and I actually work with acid nearly everyday, the most obvious answer to me was to lower my water change water in a bucket with dilute pool acid (hydrochloric acid or HCl) before adding it to the tank. The reaction between carbonate and HCl is-

H+ + HCO3- --> H2CO3 --> CO2 + H2O

The left over products are just water and CO2 that gets gassed off. So completely safe for fish.
So April, could you help us understand if there is a general formula here for water treatment? The question is, given a volume of water, with a certain KH, how much HCL to add to lower KH to the goal.

I have a 50 gallon drum of water with an air stone.
The KH is 8 degrees
I have 38% HCL (Concentrated HCL)
I want the KH after treatment to be 2 degrees

How much HCL to add to the barrel?
 
  • Thread Starter

Aprilbeingbasic

Member
Frank the Fish guy said:
So April, could you help us understand if there is a general formula here for water treatment? The question is, given a volume of water, with a certain KH, how much HCL to add to lower KH to the goal.

I have a 50 gallon drum of water with an air stone.
The KH is 8 degrees
I have 38% HCL (Concentrated HCL)
I want the KH after treatment to be 2 degrees

How much HCL to add to the barrel?
I have read this quote, from some old fish farmer page

"10 mL of 8.7% HCl acid will reduce 425 gallons of water KH by 5 ppm."

However, I dont think it matters. Dilute the acid, add a few ml. Test ph/kh in 20minutes and see what it reads. Then you can have an idea of how much your individual water buffers. No point giving people formulas when everyone has different tap water parameters.
 

Cherryshrimp420

Member
Nothing wrong with using HCl to lower the pH. Telling internet strangers to handle HCl is a problem though
 

kellerjake2004

Member
I agree with cherryshrimp420 using HCl to lower ph isn't a bad idea but the problem comes in after someone sees the article "lower your aquarium ph in 1 easy step!" and suddenly there's an influx of people adding HCl straight into their betta tanks
 

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