Calculating CO2 need

[beviking]

Does the present amount of CO2 in a tank (using Kh vs. pH) need to be accounted for before adding CO2? Going for a target of 15ppm of CO2, I already have 8-9ppm (according to the chart), so would I need (on a 90g) 2-3 DIY bottles to get to 15 ppm?
I know DIYMatt uses 2 bottles on his 75g with success.

[Slappy*McFish]

Pretty much yes, to a point..it always boils down to your pH and KH levels. Those CO2 charts are really calculating carbonic acid concentration, not free CO2 levels. So, it is possible to have a carbonic acid concentration more than 30ppm without CO2 injection, but the acidity of the water will be most unpleasant to the fish.

This article is one of the easiest to understand.
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

[promethean_sprk]

Checking out that link, my 1gal cider jug CO2 reactor brings my co2 from 3.5 to 12ppm in a 55g. So maybe 2 or 3 cider jugs would be good for a 75? Using several jugs would result in a more constant value because you could change 1/week.

[DIYMatt]

Exactly, what they said. I had my 75g up in the 12-15 ppm range with one 1 gallon bottle, but it wans't very steady and it didn't make two weeks. So, I would suggest a two bottle setup for your tank also. I use the jello method and slightly differing recipe than others. But, there are other factors play on the CO2 output of yeast generators, like temp. My hot MH ballasts are in the stand with the bottles giving them a little boost. PM me and I will send you my recipe if you like.

[beviking]

Thanks all. Um...DIYMatt...you already sent it to me...thanks again!;)

What about this from Chuck's Page...

"NOTE: If you aren't adding CO2 to your water, and the CO2 level based on the pH and KH indicates more than 5ppm, then it is very likely that some other buffer (such as phosphate) is present in your water. In an inhabited aquarium, the amount of CO2 produced by the fish will not have an effect on CO2 levels in the water. Any excess CO2
created by fish will dissipate into the air, leaving a fairly constant CO2 level of about 3-4ppm. If you test your pH and KH, and without adding any CO2, the chart says you've got 20ppm CO2, don't believe it."

I haven't measured anything yet except pH and Kh which are 7.4 and 130-150ppm (according to water authority)which gives me 8.7-10ppm according to the chart. So I don't really have that much??? Obviously I need my own testing kits, just trying to get it all down b4 diving in!:)

[happychem]

What Chuck is saying is that KH isn't just Carbonates, it's also Phosphates and possibly other chemical species that can affect KH.

When he mentions the fish vs. CO2, that can be a little tricky to get your head around. If you imagine an empty aquarium, just water, no fish or plants, there will be a certain amount of CO2 in the water, based on the temperature of your water, this is your equilibrium CO2 concentration, it's as much as your water can hold. If you add fish to this, they will add CO2 to the water through respiration, but if there's nothing to take up the excess (like plants), it just goes up into the air, for the most part. Some small percentage can go into carbonates and bicarbonates.

Basically, when you're adding CO2 from outside (like a DIY bottle or a compressed tank), you're forcing the water CO2 conc. up higher than it would "normally" be. This is why surface agitation is such an issue. You've forced the water to dissolve more CO2 than equilibrium, but return to the air through a 'stagnant' surface is relatively slow. Constant surface agitation speeds it up quite a bit.

As for how much you 'actually' have, a test kit may have the same problem as the chart, but I don't know what exactly CO2 test kits measure.

Hope this helps, PM me if I'm not clear, I do tend to ramble

[beviking]

Thanks happychem! I get the equilibrium thing, even additional CO2 is temporary, I just am confused about...

"If you test your pH and KH, and without adding any CO2, the chart says you've got 20ppm CO2, don't believe it."

So why have the chart then?
:confused:

[anonapersona]

That KH and pH is probably straight out of the tap, at which time there IS excess CO2 in the water.


Draw some water and let it sit for a day or two, then retest the pH, the KH should be the same as before but the pH will rise. If you follow the chart over along the KH line to a CO2 level of 3 - 5 ppm, that is the likely end point for your aerated water to test the pH at.

[plantbrain]

Beviking-
I'd suggest a minimum of 3x2 liter bottles, 4 would be better for a 90 gal.

You can see the www.sfbaaps.com site under "gallery" of a 90 gal I did with DIY CO2.
It was work and took s afair amount of time, if I forgot things would go sour fairly quick.

While, you can do it with DIY, I'll tell you after a close to a decade of usage, moving up to gas tanks is worth EVERY penny.

I would suggest you make a CO2 reactor such as the one Ganazafar Ghori itemized with parts, cost about 10$, will work great for most any tank.(search archives here or goggle)

Also, shoot for 20-30ppm(high range is better), NOT 15ppm CO2.
If you can, use 1 gal jugs, hardened [plastic juice bottles work best IME, or glass carboys.

Change 1/2 the set weekly, alternating, also place the brew on top of light ballast if possible, this will accelerate the CO2 production when you need it most.


So for your tank's KH, keep adding CO2 till you can hit a pH of 6.9 or so. That's all you need to do to have good CO2 in the tank.

Double check the am pH/pm pH for the tank to make sure things are going along well. Check to make sure the pH is not too far off this target of 6.9.
You can get into CO2 theory as much as you want to, but "the how" is easy and straight forward.

Regards,
Tom Barr

[happychem]

I've read, and agreed with, some articles that greatly malign the use of KH and suggest that it would be more accurate (and less confusing IMHO) to call it "Alkalinity" instead.

CO2 is different from other gasses because it not only dissolves in water but ,to a small extent, reacts with it. I don't know how familiar/comfortable you are with chemical equations, but it may make things more clear.

When CO2 enters the water (H2O) the following occurs to a small extent:
CO2 + H2O = H2CO3 (carbonic acid)

Carbonic acid can further dissociate into:
H2CO3 = H+ and HCO3- (bicarbonate)

This is where pH starts to play a role, and where CO2 affects your pH. One further step also occurs.

HCO3- = H+ and CO3-- (carbonate)

The extent of all these steps is governed by pH and the amount of CO2 in your system.

KH or Alkalinity is the measure of the negative charges resulting from carbonate, bicarbonate, and phosphate. There are others that can also influence it depending on your local water supply, but those are the 3 biggies.

The chart assumes that you've got no phosphate, if you've got a lot of plants and don't overfeed, this is probably an okay.

If you've got no phosphate, then:
KH=2xcarbonate + bicabonate
As mentioned above, the quantity of these is determined by CO2 and pH. So if you can measure KH and pH, you can calculate CO2.

You could technically correct KH using a phosphate measure, but that's likely more complicated than you (or anyone) wants to get into.

[beviking]

anonapersona, the pH as stated is out of my tank which has been out of the tap for years, given weekly water changes.

happychem, I loved chemistry, just didn't magor in it. BTW, are you in 10th or 11th grade?;)

Tom, I am seriously considering pressurized tank if I do go CO2. I'm still trying to decide if I want CO2. Thanks for the link!

Thanks all for the replies!
So why do we have the chart then?
:confused:

[happychem]

YIKES! 10th or 11th grade! Not for about 8 years! Glad to hear that you're cool with chem. I did major in it.

[promethean_sprk]

I think peat may also throw hardness/ph relationship out of whack by releasing tannic acid.

I've read that having ph below 7 will convert ammonia directly into ammonium, which plants can use for nitrogen. One wonders which scenario plants are better at outcompeting algae in, nitrogen from nitrates, or nitrogen from ammonium?

[plantbrain]

Beviking,
A moderwate to low light trank with gas CO2 will give you good plant health and slower growth, will be easier to dose(if you mess something up, skip a dosing etc there is generally less issue).

This is about one of the better combo's out there.
I'll tell you that gas CO2 is one of the best all around things folks can do if they go the carbon enriched route.

Regards,
Tom Barr