Hm. I'm still somewhat confused on this... Since the CO2 is being dispersed all over the system (by the flow doing a loop around the aquarium) how does the density in different parts of the aquarium change? Wouldn't the diffused CO2 eventually distribute itself evenly throughout the system, or is it just that gaseous CO2 is more easily used by plants and results in better growth vs diffused CO2? Could you avoid this issue by just skipping the diffuser and going with something like a reactor?
I may need to draw a visual aid, don't worry, it's a bit of an advanced concept, and in a nano tank it isn't as critical due to size and scale, you see it much more readily in larger aquariums where the co2 has to travel farther (you begin to really see the effects first hand in 10-20 gallon aquariums).
Here's the 411:
I ended up drawing a picture (edit, it's on the bottom).
Example: Filtration is on the left (intake and output) and the diffuser is on the left (as pictured in my nano). What this means is that the greatest density of water saturated with Co2 is "pushed" by the flow to the right
side of the aquarium.
Think of Co2 as a consumable, limited resource (i.e. we only have X amount in the aquarium, and won't have more than X unless we add more).
As X (water saturated with the appropriate amount of Co2) is moved by the flow from left to right, the resource is being consumed. So we have X - consumption.
Visualize the flow of a nano lily pipe modeled like the Mini P-1 and the regular P-2. It creates a jet stream that hits the opposite pane of glass.
You can follow the flow circulation by looking at how a lily pipe moves water, but if you don't have one, you can look at the picture below and picture this: out of the lily pipe water follows this path: it goes straight across (left to right), and when it hits the right pane, the water flow goes in two directions: down towards the substrate (so the water flow goes down along the right side) and then diagonally dispersed evenly across the whole right panel.
Now, as water is traveling, Co2 is being consumed by the plants, this means that the areas of greatest flow receive the most Co2
. So, Remember, X is a consumable resource and if we say that all plants across the front (concentrated flow, 1 flow stream) is being -1 Co2, and the right panel, (being -5 Co2 spread across 5 flow streams), by the time the Co2 has gone across the right panel we have:
X-6 = Co2 left available in the water column
Which means that, if our total value of X is 10 (these numbers are random for visuals), then X-6 = 4, and we have 4 Co2 left. That means that 4 Co2 is being swept away for the entire remainder of the aquarium, the back panel and the left panel.
Now, over time we have Co2 that escapes the pattern and builds up to saturate the total water volume
with Co2. Which means that over time we have a build up of say, .5
Co2 after displacement (loss of Co2) of 1 (again, these numbers don't mean anything except as a visual aid).
The important thing to know that, then due to where the flow is going the highest volume of Co2 in the water is against the opposite panel of the Co2 diffuser.
This means that plants on the opposite side
of the Co2 Diffuser grow faster, greener and healthier because they receive the greatest volume of water saturated with Co2.
There are two methods of getting adequate Co2 into an aquarium:
1. Flood the aquarium with Co2.
This is not advised since too much Co2 leads to oxygen shortage which harms fish, inverts etc and harms the beneficial bacteria. Good for a short term solution, but long time you compromise yourself by cutting yourself off at the ankles. (Fish actually contribute greatly to the health of a planted tank via waste, etc they are a huge part of the cycle).
2. Manipulate the positioning of the diffuser.
The best method: physical manipulation in response to how plants are growing by being able to switch the positioning of the diffuser and "target" areas for growth. In larger tanks, this usually translates to having two diffusers, one on each side of the aquarium (typical of 4ft - 6 ft aquariums)
Reactor vs. Diffuser
I will always argue against a reactor since you need to compensate with extra Co2 and you're limited to only option #1. You're more or less fixed on where you're going to put your co2 to plants, the same phenomena occurs as with a diffuser with flow.
"But, I could just move the filter couldn't I?"
Yes, you could, but you wouldn't be able to manipulate flow AND diffuser positioning for the BEST results dependent on your plants.
A third solution is to put the highest demanding plants on the opposite side of the diffuser (in my case, the most riccia and HC is on the right), and the ones that have the least demands (mosses, etc) on the left, (maybe now my layout will inspire an "ah-ha" moment, for being both aesthetic and practical). This is best used in Nano aquaria where their small size lends themselves well to this design.
P.S. if you've found these techniques valuable, help share the information with new comers by linking back here in your own journals when you use my techniques!