In defense of 15ppm, (not PPS Pro or rather PMDD instructions with a little PO4 added(you are welcomed to research this point)), 15ppm works very well if and only if you are limiting the rates of growth via the light or the nutrient nutrients, or both.
Example: 15ppm is adequate if you use say 1.5 W/gal of light and have non limiting nutrients.
It is also adequate if you use say 1 w/gal of light, then add a few hours, say 3 hours, of 3 w/gal then stop.
Another example where 15ppm is fine:
Strong PO4 limitation. This obviously is the linch pin slowing plant growth is this case. Adding all the CO2 in the world is not going to resolve the rate of growth nor increase CO2 critical demand if you are strongly PO4 limited and adding a small amoutn each day relative to everything else.
You could add 2-3-5X more CO2 and it would not matter.
Just because a few folks found they can use less CO2, is no surprise.
However, you and virtually everyone else in the web plant community knows that accurate measure of CO2 is a huge issue.
So who the heck knows if they really had 40-50ppm vs 30ppm or really have 15ppm of CO2 at all............
There's a huge amount of doubt with the measure of CO2 within the error ranges suggested.
I've found resolution in that.
I am going to use a LCO2 meter(these are $$$$), as I am pleased with the DO meters that use the same technology.
This provides accurate measure, likely about 1ppm error and does not rely on drop checker color, pH, or KH and has fast response times(>10min).
That + Licor light meter, and LDO O2 meter will allow much better analysis.
The other factor to consider is while water columns might be limiting or appear so, the sediment often is not. an example is ADA's sediments and the water columns appear limiting and very lean, but overall, the system is quite nutrient rich.
And getting back to 15ppm, you can see that the ADA measures cannot plausibly add to 15ppm given the pH/KH combos in any know ranges.
But if you consider the low light(1 w/gal, sometimes under or over a little), + the high noon blast for 2-4 hours, 15ppm does not seem particularly limiting at such light levels.
Failure to consider such global understandings about what grows an aquatic plant, can easily lead down a long path of poor assumptions. I see folks often change one thing and assume it does not affect the other
Do not do that!
You end up assuming things are the way they are for the wrong reasons.
Which is worse than algae
Same deal with a drop checker, they work, they are slow to respond(2-3 hours or so), they require you to tell/eyeball btw 0.2pH color units.
They have trade offs, but they are better for many and cheaper than pH meters etc.
I use higher nutrient levels so I do not have to wonder if I have other interactions that slows down CO2, or other growth parameters. I also use higher light levels to see how that influences nutrient demand and CO2 demand, there is a trend and it shows up in all research done on aquatic plant growth and limitations.
By providing non limiting growth parameters except for the parameter of interest, is the basic method to investigate plant growth. You may try and combine 2 or more parameters, but the results become harder to tell apart, and when folks cannot set up and single parameter test well, how are they going to do more complex stuff?
Regards,
Tom Barr
