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CO2 Questions on water parameters, inhabitants, and power.

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ive decided to take the leap and set up one of my heavily planted tanks with a CO2 system. For no reason other than just wanting to try it out. I’ve gone 10 years without CO2 and just want a change.  I’ve done some research but still have questions. I hope someone on this forum can help answer them. The general “internet” is making me fearful of CO2! 

1. will CO2 have an affect on any major water parameters- such as PH? KH? GH? I have very hard water, my PH is usually between 7.2-7.6 and I have like low to moderate KH. 

2. are there any legit dangers to the inhabitants? The internet has lots of fear mongers saying it can kill all of your fish if you inject too much, but how much is too much? 

3. what happens if the power goes out? I purchased a regulator that will be  plugged into a timer/wall outlet, but I don’t have any back up power units and don’t intend on buying any**
Is it okay for it to just not run if the power goes off for a couple days at a time? Will there be major plant or fish deaths if so? And is there anything I have to do to the regulator or the tank in a power outage? 

***it’s just too expensive to have elaborate back up power. I do run battery operated air supplies and have a few USB run air stones that I use smaller battery packs and solar chargers for, but I don’t intend on getting like a big generator or anything too elaborate. 


thank you in advance 🙂 

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Disclaimer: I have not tried CO2 in any of my tanks yet, so most of what I am about to say is secondhand knowledge. Hopefully someone else can affirm or disprove what I say with their own experience.

1) Yes, CO2 will affect pH, making it drop (more acidic). This is actually something happening at a large scale in the world’s oceans right now known as ocean acidification. 

2) The reason why there is so much fearmongering is because injecting CO2 can make you susceptible to pH crashes. Because you are driving the pH down, the buffering components in your water (things that make up KH and GH) get used up. When you run out of those buffering components, the pH will plummet, making the water extremely acidic and deadly to anything living from fish to bacteria. What people usually do is buy a pH controller. It basically monitors your pH and shuts off when there is a pH crash, just like how a heater is supposed to shut off when the water around it gets too warm. 
I remember from a livestream that Cory also suggests running an air stone in tanks with CO2. The reason is that the water can get so saturated with dissolved CO2 that oxygen becomes more scarce, and the fish will essentially  start dying due to a lack of oxygen.

3) I think it depends on the plants you keep in the tank. If you have plants with high nutrient, light, and CO2 needs, a couple days may be all it takes to disrupt the balance and either cause a melt OR an algae outbreak. If you have low tech plants, they should fare better. Assuming you dial back the light and fertilizers when the power goes out, I think it should be fine. That being said, changes happen a lot faster with CO2, so finding the right balance between CO2, nutrients, and light is key. For the fish, just toss an air stone in there and make sure oxygen levels don’t drop. 

Hopefully that gave you some idea of how to proceed and maybe next steps/other questions. 

Edited by AnimalNerd98
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CO2 will lower your pH while it's active, this is normal and desired. If you do 50% weekly water changes, like you should do when running CO2, you should have no problem with GH, KH and pH. FYI, I run 0dKH.

There is a danger, you can add too much CO2 and kill your fish, but taking a day to adjust CO2 properly will prevent this from happening. You want 30ppm of CO2. Also, purchase a good regulator and you should be fine from failures. 

If you loose power, your CO2 simply turns off, you would then need to calibrate the timer it's plugged into. Nothing will die because CO2 is off, but it will upset the plants, they love CO2.


FYI, all in one fertilizers will be challenging, it's best to go dry fertilizers.

Here is a link to every question you will have. Going CO2 is absolutely amazing. Don't be scared.


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I'll preface that what I'm gonna say is mostly opinion but I'll share what I've experienced so far. I'll try to share some points of concern for you to consider just so you know what you will need to keep an eye out for while injecting co2.

But a lot of concerns that you have at the moment mainly hinge on scenarios where folks try to walk the razors edge and dump an enormous amount of co2 trying to compensate for ridiculously high lights or demanding plants. Since I don't think that you will be injecting co2 to those extremes at the start, you wont be posing any issues pertaining to your fish. But very generally, as long as you monitor your fish whenever you make co2 adjustments, you shouldn't have a problem.

My main advice is to take things slowly and make small incremental adjustments over time. Since you sound quite experienced I'm sure you'll see major improvements even with the tiniest amount of co2. A little will go a long way if you can create favorable conditions.

1. If you can provide your specific gh/kh parameters we'll have a better idea on what kind of issues you should look out for but yes, co2 will affect all three one way or another. Since co2 is acidic, driftwoods and certain stones can raise or lower gh/kh as they are being dissolved in the water.

Understanding the relation ph/gh/kh and how they work together is important but kh should be your first concern of the three. You'll find ph fluctuates throughout the day which is normal but the lower the kh of your tank is, the more prone you are of dropping your ph too low. Ideally you would want some kh, at least 3dKh to give you some room for mistakes.

The kind of fish you have will also dictate how much co2 you can inject into your system. Introducing co2 slowly would be key to allow your fish to acclimate to the adjustments.

2. As said before you do run the risk of gassing your fish but it mostly happens if you are carelessly cranking the co2 or if you buy a low quality regulator. 

You'll hear 30 ppm co2 as a benchmark of how much co2 you should have in your tank but its not really a good measurement. Aiming for the 30ppm range especially when you are new is where you'll start getting into trouble. Unless you are trying to grow the absolute most demanding plants, you'll have good success with even half that amount.

But if you are so inclined you can find ph/kh/co2 charts online or use a drop checker for co2 measurements but understand each of these tests have their own pitfalls that only tell you half the story. There are so many interactions happening in your tank that its hard to account for factors than will skew test results. Personally I think you are far better off looking at the response from your plants and fish instead of trying to chase numbers.

A few things you should consider is that temperature plays a role in how well gases can be dissolved into water. Warmer water can hold less o2/co2 than it can when its colder.

Circulation is much more important than a high concentration of co2. Co2 is useless if you don't have an adequate delivery system to transport co2 and nutrients around the tank.

Good surface agitation allows for gas exchange that helps prevent co2 building up to a lethal level. Keeping in mind that you can kill your fish with co2 poisoning even if your plants are pearling and you have high oxygen saturation.

3. If your regulator is hooked up to a solenoid, then I wouldn't ben concerned about power outages. I don't even think your plants will be affected by a blackout for a few days since your plants won't need co2 if lights are off. Now if your power is out for like a week you might get a little melting but you'll be back on track in no time with co2. 

You may not even need it, but having an air pump that can plug into a power bank would be nice to have in case.


Sorry if I'm being way too elaborate but the last thing I want to be is vague/unclear in this matter. I'm not trying to scare you in anyway just share some issues I ran into. Definitely take everything I said with a grain of salt and use my topics to direct you in areas you should research. I'm pretty biased on some of my points and may very well be wrong but my goal is for you to know what to look into so you can come to your own conclusions. I'm sure you know by now how hard it is to parse through information online.

If you haven't already, it might be fun to document your progress in a journal so we can all follow along. Good luck!

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  • 5 months later...

@Mmiller2001i recently just added CO2 to my 20L and I saw that you mentioned doing regular 50% water changes. Why is that? 

I am finding a lot of resources about how to set up CO2 in your tank but not having much luck on what you should do to maintain a CO2 injected aquarium after you’ve added it. 

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On 12/22/2021 at 9:27 PM, Greg H said:

@Mmiller2001i recently just added CO2 to my 20L and I saw that you mentioned doing regular 50% water changes. Why is that? 

I am finding a lot of resources about how to set up CO2 in your tank but not having much luck on what you should do to maintain a CO2 injected aquarium after you’ve added it. 

Doing, at least, 50% water changes allows you to reset nutrients in the tank, reduces organics and is an overall important component of good husbandry. You can also use the 50% changes to target nutrient ppm as you can never have more than 2 times the amount you dose.

Example, if you have an aquarium and dose 10ppm of NO3 weekly, do 50% changes, you can never have more than 20ppm NO3.

This is a good resource for maintaining a CO2 tank. 


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