# What size co2 tank?



## BigChuckP (Oct 8, 2005)

I want to buy a co2 tank for my 75 and was curious what size tank I should go for. I was thinking a 5#, but want to know how long it would normally last. I don't want to be refilling it on a weekly basis or something like that. 
Thank you for any replies.


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## MatPat (Mar 22, 2004)

With a KH that averages 5 dKH my 5lb tank lasts about 6 months.


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## BigChuckP (Oct 8, 2005)

MatPat said:


> With a KH that averages 5 dKH my 5lb tank lasts about 6 months.


How big is your tank?


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## AaronT (Apr 26, 2004)

MatPat said:


> With a KH that averages 5 dKH my 5lb tank lasts about 6 months.


KH has no relevance as to how much CO2 one must add. You would add the same amount of CO2 to a tank with 2 KH as you would a tank with 8 KH and so on.

A 5 lb. tank will last at least 6 months and longer if you use a solenoid and turn it off at night. I had a 10 lb. tank on the same size tank run 24/7 for a year.


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## JanS (Apr 14, 2004)

grandmasterofpool said:


> KH has no relevance as to how much CO2 one must add. You would add the same amount of CO2 to a tank with 2 KH as you would a tank with 8 KH and so on.


I must be missing something here, Aaron. :???: I have a higher KH (9) and I have to pump in more C02 to keep my levels at a consistent rate. I use a controller, and obviously a higher KH is going to be more resistant to bringing my pH down to the desired level than a lower KH that will change much faster. I know you need a lower pH with a lower KH, but I don't think the C02 consumption has the same formulation when comparing KH. What am I not seeing? :-s

BigChuck, a 5# would work just fine on a 75 gallon and like the others said, probably last over 6 months, but if you have the means, it's much more economical to go with a 10#. Around here, Airgas only charges about $2 more to fill a 10# opposed to a 5#, and when you consider the hazmat charge per tank, you're really saving to get twice as much for the same hazmat fee.


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## AaronT (Apr 26, 2004)

JanS said:


> I must be missing something here, Aaron. :???: I have a higher KH (9) and I have to pump in more C02 to keep my levels at a consistent rate. I use a controller, and obviously a higher KH is going to be more resistant to bringing my pH down to the desired level than a lower KH that will change much faster. I know you need a lower pH with a lower KH, but I don't think the C02 consumption has the same formulation when comparing KH. What am I not seeing? :-s


Jan,

That shouldn't be the case. For example if you have a tank with a KH of 3 and a 2 bubble/sec CO2 rate gives you 30 ppm CO2 levels then upping the KH of that same tank should not result in a need to adjust the CO2 bubble count. In other words it is not an exponential relationship. At least, this is my understanding. Someone please correct me if I am wrong.


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## czado (May 26, 2005)

Jan,

It may be easiest to look at relastic numbers, with the assumption that no other buffer is messing with CO2's relationship with KH and pH.

CO2 ppm = 3 * dKH * 10^(7-pH)

Constant CO2 from aged tap of 3ppm and 30ppm CO2 after injection:

Example 1
1 dKH
Tap pH of 7.0
pH after injection 6.0
pH change 1.0

Example 2
3 dKH
Tap pH of 7.5
pH after injection 6.5
pH change: 1.0

Example 3
10 dKH
Tap pH of 8.0
pH after injection 7.0
pH change: 1.0

HTH


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## BobAlston (Jan 23, 2004)

Buy the biggest co2 tank that will fit where you are going to keep it. I have my 5 lb co2 under my 55 gallon tank. It lasts about 6-8 months. wish I had a 10 lb tank but not sure if it would fit. It often costs very little more to fill a larger tank than a small one.

Bob


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## Raul-7 (Feb 4, 2004)

I currently have a 15lb tank on a 37G Cube. I got mine used, but it's tested and primered (June 2005) for $36. Try asking around welding supply shops for a good deal.


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## Bert H (Mar 2, 2004)

> Buy the biggest co2 tank that will fit where you are going to keep it.


I agree with Bob. Also check at fire extinguisher companies, that's where I got mine.


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## BigChuckP (Oct 8, 2005)

Found someone through the internet who is selling a 5# tank for 30 bucks! Picking it up this weekend! Can't wait to set this up and see my tank explode with growth...that is if I do it all right


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## toofazt (Aug 9, 2005)

Good deal, looking forward to some pics


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