mikeappleby said:
Lots of us experience extra pearling after a water change. This is often explained as the water from the tap being 'saturated' with CO2,leaving the 'extra' O2 from photosynthesis more visible.
Err..Mike I think you are mixing your marbles. To get to the bottom of the story you need to first understand the concept of partial pressure. Think about your bicycle or car tires, and think about
all the different gasses that are contained in air ; O2, N2, CO2 and so forth. When you first install the tire the quantity and relative ratios of each gas in the tire is the same as that in the atmosphere. The amount of pressure that each gas exerts against the wall of the tire is different, so for example, most of the air is made up of Nitrogen (N2) so that most of the pressure that the air exerts against the wall of the tire is due to the pressure that the Nitrogen is exerting against the tire wall. So in a flat tire, where the pressure inside the tire is the same as the pressure outside the tire, the pressure of the air is 1 Atmosphere (14.7 psi) and if the air is composed of 80% Nitrogen then that means that 80% of the pressure on either side of the tire wall is due to the pressure exerted by Nitrogen, i.e. 0.80 Atmosphere (11.6 psi).
So now try to figure out what happens when I pump the tire up to 2 Atmospheres higher (30 PSI) than flat. I've now put 2X as much extra mass of air in the same volume, but since I used air, the ratio of the gasses is still the same. Nitrogen still exerts 80% of the pressure against the inner wall of the tire 2Atm*0.8 = 1.6Atm = 24PSI due strictly to Nitrogen alone.
now lets get to your tap water. The only way I can deliver water to your tap from miles away is to pressurize the water so that it will flow through pipes and exit when you open the valve right? So tap water is pressurized to some value determined by your municipal water supplier. Therefore any gasses already dissolved in the water at the time of pressurization necessarily also get pressurized and so there is O2, Co2, N2 and other gasses trapped and pressurized inside the volume of water being delivered to your tap. These gasses are at a higher pressure than the gasses that are in your aquarium. Gasses dissolve better when they are under pressure and when they are at a lower temperature, so when you open the tap and allow the water to flow into your cup there is a higher concentration of all gasses in the water in that cup than there would be of those gasses in the atmosphere.
If you allow the cup of tap water to sit, then the water and the gasses in it are no longer under the same pressure that they were in the pipe. they are now under atmospheric pressure (14.7PSI). can you see that this is like opening the valve of your tire? Gasses which were under pressure and were highly dissolved due to that pressure now escape from the water into the lower pressure of the atmosphere. This doesn't happen quite as fast as the tire valve though, it may take 1/2 an hour or more depending on the relative temperatures of the room and the water.
So when you put tap water into your tank directly you are also putting high pressure CO2 into the tank in exactly the same way as if you were injecting CO2 into the tank. In fact, whatever the pressure of the tap water was, it's as if you were injecting the CO2 at a needle valve pressure of the pipes in your house. So if you tap water is pressurized to 25PSI then adding this water to your tank is kind of like setting your CO2 needle valve to 25PSI. The difference being that you are not delivering the same volume or mass of CO2 as you would be if you set the needle vale that high, but the principle is the same. After 1/2 hour or an hour the gasses escape, but the extra CO2 that you had put into the tank under pressure can find it's way into the plants an this extra CO2 is used in photosynthesis, which produces O2 at a higher rate in exactly the same way as if you had the needle valve set at a higher pressure. Again, the volume and mass of CO2 that you are adding with tap water is not the same as adding pure CO2 under pressure via the needle valve but the effect is the same.
Sometimes, depending on where you live and depending on where the municipal supplier gets the water from, the water travels through underground areas where bacteria are producing CO2 as a result of their respiration, or where CO2 concentrations are high due to geological or other biological processes. The water then absorbs extra CO2 which will be delivered to your tap after it is pressurized.
OK, so now lets look at water changes. What happens when you remove water from the aquarium? The plants are then exposed to air, which, although is only at atmospheric pressure, and which is only composed of say, 0.03% CO2, the fact is that this CO2 finds it's way into the plant a lot quicker than CO2 in water can find it way to the site within the plant that uses it. There are many barriers and obstacles acting against CO2 entry into the plant while under water, but when you drop the water level and expose the plant tissue to air all these barriers are immediately overcome - the tissue density is no longer an obstacle, the bio-film covering the surface of the tissue is no longer an obstacle, and the poor ability of gasses to even dissolve in water is no longer an obstacle.
Now, you fill the tank with CO2 enriched water from tap and the leaves which were exposed to air now have effectively taken a gulp of atmospheric CO2 - with air still trapped in the tissue and now being pressurized by the added water, so the sudden increase in CO2 availability increases the photosynthetic rate. The increase in the rate of photosynthesis is
always accompanied by an increase in the Oxygen production rate. After a few hours the extra amounts of gasses have all escaped.
Also, it should be noted that plants always perform better in clean water with lower levels of organic waste. These waste products add to the biofilm on the surface of the leaves and this biofilm is one of the barriers to the permeability of the leaf to accept CO2 and nutrients. So it is always in the interest of a high energy tank to do as many water changes as possible and as large a volume as possible.
mikeappleby said:
...If this theory is right then that would explain why frequent massive water changes are (a) good for plant growth (maybe particularly when settling in a new tank), as they actually increase plant photosynthesis for a few hours period after the change...
...Is quite important because theory two would explain why big frequent water changes can sort out a tank that's struggling, and mean that if you're trying to bed in a new difficult plant that requires lots of CO2 (like HC for example) then it might be worth upping the water change frequency...
CORRECT!!
mikeappleby said:
... and (b) good at killing algae like bba that are CO2 related.
No. You need to stop with the killing. That is why people fail. This is never about killing. Do not think in terms of killing algae, but instead think about preventing them from blooming by using techniques designed to optimize plant health.
mikeappleby said:
I've never really checked to see whether a drop checker changes after a water change - has anyone noticed a big impact here?
You cannot notice this because the mechanism by which the dropchecker works is too slow.
Cheers,
the co2 content has nothing to do with it 
its all about the o2
