Ph Question

[discus dreamer]

Hi
I have two tanks and one (big tank) has a ph on the high side 7.2-7.4 the other (small tank) is on the low side 6.4
Obviously i use the same tap water for both so the differences are: big tank has sand and gravel, small tank has just gravel. Big tank has three pieces of bog wood, small tank has none. Big tank has pressurised C02 small tank has a yeast type C02 kit. Big tank has two lumps of quartz type stone and two pieces of slate. Both relatively heavily planted.
Any ideas about what is making the big difference here? I am about to switch the small tank to just sand and add a piece of bogwood so it will be interesting to see if that makes any difference.
I am thinking of getting discus at some point so it would be good to know how I can get my big tank to stabalise at a lower Ph. I think the water is relatively soft.
any help here would be appreciated
thanks
DD

 

[BigIan]

Many Many MANY MANY Possibilitys there...

The sand may not be inert sand and so acting upon the ph of your tank.
What sizes are the tanks? as the smaller tank may have a greater ppm of co2 in it due to the smaller volume.

 

[paulthegreat]

The bog root should reduce the pH and the carbon dioxide should be reacting with the water to produce acid so the large tank should have a very low pH. The sand could be coral which breaks down to give an alkaline reading which can be hastened by the acidic properties of the aforementioned. You have so many variables though that it is hard to say for sure.

 

[cardinal sinner]

since i put bog wood & co2 in my tank the ph has gone from 8.4 to 6 im well impressed

 

[Bignose]

as the smaller tank may have a greater ppm of co2 in it due to the smaller volume.

Ummmm, no. ppm is a concentration measurement, and so is an intensive quantity. What this means is a quantity that does not depend on the size of the system.

Let me give you an example. Let's mix up some lemonade from frozen concentrate. You mix thoroughly in a jug -- you mix it so well we'll even call it perfectly mixed. Then you pour yourself a glass from the perfect mixture. Is the concentration of lemon to water in the glass any different than it is in the jug? No, because lemon to water is a concentration.

The opposite of intensive quantities are extensive qauntities. Is there less total amount of lemon (not concentration of lemon to water, just total lemon) in the glass than the jug? Yes, because the glass is smaller. Extensive quantities do depend on the size of the system.

So, all that was to explain that just because a tank is smaller is no reason whatsoever for the concentration of CO2 in it to be less. The total amount of CO2? Yes, that will be smaller, but not the ratio of CO2 to water, which is the concentration of CO2 typically measured in ppm.

==============

Now, what probably is happening to the OP, because the the tank is smaller, it will be more affected by the same total amount of CO2. That is, a smaller change in CO2 will result in a larger change in pH than with the large tank. This will make the smaller tank's pH harder to control.

I suspect that this is what BigIan meant by his statement.

Seems to me you need a finer control on the small tank to keep the pH more steady, and more frequent water changes would help refresh the buffers in the tank to help keep the pH more steady. You may consider purchasing a chemical buffer from the LFS, though I'd look pretty carefully at the ingredients list as a lot of those buffers lead to an overwhelming amount of algae. Though, I guess if your plants are growing very quickly, they will probably out compete potential algae.

 

[xoedusk]

I hope someone here can pick through my understanding of chemistry:

I would expect a smaller tank to have a higher CO2 conentration, given everything else is equal, as BigIan said. If you have 2 bubbles per second coming out into a reactor in a huge tank and a small tank, they are both going to mix the CO2 with the water the same (forgive me, I don't know the right term). So same CO2, less water in the smaller tank. Same CO2, more water in the big tank.

At least that's how my brain sees it. I suppose there comes a point when the rate of CO2 loss (via atmosphere) is equal to the CO2 gain (via bubbler), so that plays a part.

 

[Bignose]

OK, let me take another crack at this.

If you put the exact same bubbler, with the same outputs and so on, the smaller tank would have the higher CO2 concentration. This is just simple division. Dividing the same number (the amount of CO2) by two different numbers (the volume of the tanks), dividing by the smaller number always results in the larger of the two ratios, or concentration in this case.

But, that is not the situation at hand. There are two different CO2 bubblers here, and BigIan said that the smaller tank will have more CO2 in it due to the smaller volume.

That is just not right. There are several other variables that come into play. Basically, I was just objecting being able to know which concentration is larger based on only the volume of the tank -- which just is not possible.

 

[whippsy]

the slate and other stone will make the ph go up as in mine and i couldn't put it in. the wood is good and if the sand is proper and pure then it shouldn't add to the ph. a simple test to see what is unsuitable put an item from your tank into vinegar and if it bubbles or you can see a reaction in the vinegar then dont use it