What does hydrochloric acid do in saltwater?

[keagan]

I worked for a company for a year that grows dulse and sea lettuce on land in large wood lined pits with pond liners on them. We scrubbed the sides of these tanks after lowering the water level to three feet and using pump up sprayers full of diluted hydrochloric acid. We also occasionally poured undiluted hydrochloric acid into the tanks for set periods of time.

They were under the impression that hydrochloric acid killed pest algae including ones that are free floating in the water. Can this work? If so I'm surprised it's never been used in reefing to kill hair algae.

 

[unchainedcoot]

Algae wouldn’t be the only thing it kills

 

[keagan]

Algae wouldn’t be the only thing it kills

At a high enough dose it would kill the sensitive dulse but it survived the spraying and dosing and it's pretty finicky stuff

 

[Privateye]

The problem/use of HCl in saltwater is it's a strong acid. It will affect your alkalinity as well as your pH.

I've used muriatic acid (diluted hydrochloric acid) in systems with animals to drop the alkalinity. It's kind of a two-step process, as the first thing it affects is pH. This is where the danger comes in. So when I add acid to a given chamber in the system, or even just any container of seawater, the pH will drop, which causes more of the inorganic carbon in the water to exist as carbon dioxide (and thus carbonic acid). With adequate aeration over time, the CO2 gasses-off and the pH rises. But you've lost inorganic carbon as CO2. Inorganic carbon that was previously carbonate or bicarbonate. Hopefully that makes sense. If not I'll include the pKa curve for a clearer explanation.

So while it can be useful for lowering alkalinity, the short-term effect is low pH. That's what causes things to die. Algae can be one thing that dies, but that's a really rough experience for a coral. Especially a stony coral. It's not uncommon to see the local pH drop to 5 when I dose, which is miles past "ocean acidification" levels. It takes a few hours of aeration for the pH to come back up, assuming I didn't overdose.

If the addition of HCl is low-enough that it won't impact total pH in the system by much, then it's often fine. The long-term effect is lower alkalinity. It's getting through the low pH period that's risky for desirable organisms.

I once had a customer call about a problem with his catfish pond. It was spring-fed but the water only exited through evaporation. After years, his catfish were no longer eating and could not be caught on hook and line. His pH was 9.0. I told him he could go to Home Depot, buy a bunch of muriatic acid, and add it to his pond over time. I warned him that I could not say how much he would need, but "add it slowly, distribute it well, and wear PPE". A year later he got me on the phone again with "oh yeah, you're the one who told me to add acid to my pond". I knew it was risky, but fortunately he thanked me. He got his pH down to 7.0 and the fish were acting normally. If I hadn't been the one to answer the phone, I would have wondered whether my advice killed his fish and he ghosted us!

 

[keagan]

The problem/use of HCl in saltwater is it's a strong acid. It will affect your alkalinity as well as your pH.

I've used muriatic acid (diluted hydrochloric acid) in systems with animals to drop the alkalinity. It's kind of a two-step process, as the first thing it affects is pH. This is where the danger comes in. So when I add acid to a given chamber in the system, or even just any container of seawater, the pH will drop, which causes more of the inorganic carbon in the water to exist as carbon dioxide (and thus carbonic acid). With adequate aeration over time, the CO2 gasses-off and the pH rises. But you've lost inorganic carbon as CO2. Inorganic carbon that was previously carbonate or bicarbonate. Hopefully that makes sense. If not I'll include the pKa curve for a clearer explanation.

So while it can be useful for lowering alkalinity, the short-term effect is low pH. That's what causes things to die. Algae can be one thing that dies, but that's a really rough experience for a coral. Especially a stony coral. It's not uncommon to see the local pH drop to 5 when I dose, which is miles past "ocean acidification" levels.

If the addition of HCl is low-enough that it won't impact total pH in the system by much, then it's often fine. The long-term effect is lower alkalinity. It's getting through the low pH period that's risky for desirable organisms.

I once had a customer call about a problem with his catfish pond. It was spring-fed but the water only exited through evaporation. After years, his catfish were no longer eating and could not be caught on hook and line. His pH was 9.0. I told him he could go to Home Depot, buy a bunch of muriatic acid, and add it to his pond over time. I warned him that I could not say how much he would need, but "add it slowly, distribute it well, and wear PPE". A year later he got me on the phone again with "oh yeah, you're the one who told me to add acid to my pond". I knew it was risky, but fortunately he thanked me. He got his pH down to 7.0 and the fish were acting normally. If I hadn't been the one to answer the phone, I would have wondered whether my advice killed his fish and he ghosted us!

Thanks for the explanation! I was wondering if small amounts in a pipette could be used for killing spots of algae but the ph lowering definitely wouldn't be worth it.

I was surprised they got us to dump it in the tanks and let them sit for a half hour. The dulse seemed unfazed and a small change in temp or salinity killed that stuff. The ponds were plumbed right to the ocean so the pH would've gone back to normal quickly when the pond was refilled but it must've dropped quite a bit while it was sitting. We had massive UV sterilizers as well I bet that's what actually killed the pest algae.

 

[EnterName]

You already got a great answer but I there is one tiny detail that might be worth mentioning:

They were under the impression that hydrochloric acid killed pest algae including ones that are free floating in the water. Can this work?

There is no need to kill free floating (planktonic) algae in reef tanks as it doesn't survive for long anyways:

• Skimmers remove it
• UV sterilizers kill it
• Copepods and other organisms eat it
• It has to compete over nutrients and trace elements with other organisms.

That's why people grow phytoplankton in separate containers and often have to keep it quite sterile or the culture crashes sooner or later. Muriatic acid is great for cleaning those containers or pumps though.

It's also the reason why nuisance algae/bacteria that cause issues in reef tanks are almost always benthic (living on surfaces). The only exception are short algae/bacteria blooms that die down on their own after a while.

Changing water parameters like pH or alkalinity can obviously kill benthic nuisance algae, but it will probably kill corals first.

 

[Randy Holmes-Farley Verified Community Expert]

I agree it’s not a plan to try. Hydrochloric acid is no different than any other acid added to seawater, dropping pH and alk.

 

[KrisReef]

FOr the rest of us:
AI Overview
Dulse (Palmaria palmata) is a nutrient-dense, red seaweed that grows wild in the cold waters of the North Atlantic and Pacific Oceans. Naturally salty with a savory, slightly "bacon-like" flavor when fried, it is rich in protein, potassium, iron, iodine, and vitamins B6 and B12. [1, 2, 3, 4, 5]
`
NOte: Bacon Like Flavor! Rinse well after acid washing!

 

[Randy Holmes-Farley Verified Community Expert]

FWIW, I bought a bulk pack of human food dulse online and feed my yellow tang and one spot foxface with it.,