Ph Change After Adding Air Stones

[jnms]

Hi, last night - I noticed that my fish were breathing at the top of the tank. Realising this was a sign of lack of oxygen in the water I today fitted two air stones - this however has raised the Ph somewhat.

Here are my tank stats:

Tank 200ltr

Ni - 0.0
Am - 0.0
Ph - currently 7.9 - before air stones 7.4
Kh - 13

Currently injecting DIY CO2

Fish include, Zebra Danio, Guppies, Bristlenose and Bolivian Ram.

The Guppies, Rams and Dannios seem to be fine with the change in Ph. The Bristlenose is a little unhappy about it - she is still eating and moving around the tank quite readily, but her colours have darkened a little - which is what happens when she is stressed.

I understand that a stable Ph is generally more important than a specific Ph, however I am wondering if you guys feel that the bristlenose will adapt ok to the new Ph?

Is 7.9 a little high? Is there anything else I can do to lower it a little? (At the moment I have bogwood in the tank and inject Co2)

 

[Mr Bliss]

Heya, not sure about the pH, but I'd just like to comment on the airstones:

In general, airstones won't do much for your oxygen levels but they will get rid of your CO2. If possible, I'd suggest you keep your airstones on only at night. They defeat the purpose of your CO2 injection. CO2 does not like to be dissolved in water, and will readily go all the way down to 0.5 ppm under heavy aereation (whereas the desired level for your plants is 10 - 30 ppm). The rise in your pH after adding the airstones is solely due to loss of CO2, and depending on your lights you may be in for an algae bloom.

Given that according to every reputable planted tank site, including the experts on this forum, it's practically impossible to overdose on CO2 using a DIY system (especially in a 200 litre tank!), and that CO2 does not displace oxygen in the water, and that plants do not consume significant amounts of oxygen at night, it seems likely that you have insufficient water circulation in the tank, or a surface film preventing gas exchange between water and air, or a lot of decomposing material in the tank, or a filter system that consumes a LOT of oxygen (undergravel filter?). I'd look into those before using airstones, because airstones and CO2 injection really don't mix.

 

[Colin_T]

Water can only hold a certain amount of dissolved gasses, be it oxygen (O2), carbon dioxide (CO2) or nitrogen (N). When you add CO2 you reduce the other gases in the tank. However, when the plants are given light they use the CO2 and release O2. At night when the lights are off the plants no longer use CO2 but instead use O2 and release CO2. If there is insufficient surface turbulence or an oily film on the water surface, then the CO2 levels can build up and cause the fish to suffocate.
Filter bacteria consume lots of O2 and release CO2 all the time thus adding to the CO2 in the tank. Rotting organic matter in the gravel will feed more bacteria that also produce CO2.
High water temperatures will reduce the water's ability to hold gases and warm water will hold less O2, CO2 & N than cool water.

Bristlenose catfish are fine in alkaline water but yours is probably showing stress markings due to the low oxygen levels rather than the raised PH.

As you stated, a stabile PH is more important to fish. Wild caught fishes do best in the correct water parameters but most fish are fine in waters outside their normal levels as long as it is stabile.

A lot of people have the airpump on a timer so it comes on at night and goes off just before (30minutes or so) the lights come on. However, care must be taken doing this as you can have massive PH fluctuations between the day and night and this can stress the fish.
A small water pump aimed just under the surface will often provide adequate surface turbulence to prevent the fish suffocating but not drive out too much of the CO2, thus keeping the PH more stabile.

 

[andywg]

Water can only hold a certain amount of dissolved gasses, be it oxygen (O2), carbon dioxide (CO2) or nitrogen (N). When you add CO2 you reduce the other gases in the tank.

Do you have anything to back this up? I have repeatedly read that for all intents and purposes at our usage the level of dissolved O2 is independant of the level of dissolved CO2 - raising one most definitely does not lower the other. Water can easily hold enough O2 for fish and CO2 for plants but due to the CO2 having a lower level naturally in water than desired in planted tanks it will leave the water at any air/water interface (the surface).

 

[mike os]

If I remember correctly water is capable of containing several dissolved gases at levels largely independent of other gases under normal conditions ( partial pressures of dissolved gases???)

O2 and CO2 can both reach saturation levels without affecting each other, IIRC it is only when supersaturated that out gassing of other gasses occurs, and to supersaturate you need to inject large quantities of pure gasses, not air.

Aeration by airstones has an effect on CO2 levels not because of increased O2 into the water column, but because increased surface agitation (caused by water movement around the bubbles) encourages out gassing of the CO2 to air. In fact airstones add very little of themselves, it is the same surface agitation that increases O2 levels in water, not the bubbles themselves.

with a Kh of13 your "real" ph is probably about 7.8 or so, & the CO2 is lowering it, (in water= mild carbonic acid).

I cannot really advise on CO2 levels as I have not used it for a number of years, but I would suggest that if CO2 is causing respiratory problems then you may be adding too much.

hopefully a green weed guy will be along soon to help

 

[jnms]

Thanks for the replies guys!

I have reduced the output of the airpump, done a small water change and stabalised the CO2 injection somewhat.

This seems to have brought down the Ph a little - which is now at 7.6 and I am a little more happy with that. Remains to be seen if this remains consistent.

I would be very interested to see more information on gas capacity within water if someone is able to point me in the right direction. Thanks again for the help!

 

[Mr Bliss]

The reason CO2 can kill fish isn't CO2 displacing O2 in the water, but CO2 displacing O2 in the blood of the fish, leading to acidosis and suffocation if the ppm is too high. This is commonly said to be a potential danger when you go beyond 30 ppm CO2, but some planted tank hobbyists maintain ~40 ppm without ill effects. The reduction in pH of the water can probably play a role too, for species that don't tolerate acidic water.

As to the amount of dissolved gases, here's how I understand it, although I'll start by admitting that my physics is a bit rusty:

Water will always equilibriate to a certain total amount of dissolved gases over time, Colin_T is correct about that. The proportions of different dissolved gases in water are determined by their partial pressure in the air above the water, and their individual solubilities in water, which is pretty low for O2 and N2, and high for CO2. "Saturation" in this context means the point at which these two factors balance out and the gas in water is at an equilibrium with air. The saturation point of CO2 is only ~0.5 ppm due to the low partial pressure of CO2 in the air, even though CO2 has extremely high solubility compared to oxygen; higher concentrations than this will gradually escape the water, as anyone who has ever run CO2 injection knows; it doesn't take long for your tank to settle at an ambient level of a couple of ppm CO2 (coming from the fish). Planted tanks are commonly supersaturated with both CO2 and O2 during the day, at the expense of nitrogen. Once O2 production stops and (let's assume) CO2 level is kept constant, then yes, CO2 will push a fraction of both O2 and nitrogen out of the solution, but only in the most pedantic sense, because of the high solubility of CO2:

The solubility of CO2 is ~1500 mg/l at 25C if pure CO2 is equilibriated with water.

The solubility of air (nitrogen + oxygen) is ~23 mg/l (of which 9 ppm is oxygen, 14 ppm nitrogen) if air is equilibriated with water (which pretty much happens with any open container over time).

So looking at the capacity of water to hold total dissolved gases, increasing CO2 by 1 ppm would displace 0.006 ppm (9/1500) of oxygen and 0.009 ppm (14/1500) nitrogen. If we forced CO2 to stay at 40 ppm, the concentration of oxygen would therefore drop from 9 ppm to 8.76 ppm. Fish get stressed below 6 ppm, and 3 ppm is too little for most species to stay alive. To reduce oxygen concentration by just 1 ppm we'd need to inject 167 ppm CO2; depletion of O2 in the water would be the least of our worries as the fish would die rapidly of CO2 poisoning.

edit: fixed some inaccurate wording

 

[jnms]

Thanks for that Mr Bliss, you have a wonderfully detailed way of explaining things that is easy to understand.

Really is a help - cheers...