Crazy Loaches;21172 said:If by 'naturally' you mean prior to ever injecting any co2 then it doesnt sound like your levels are anywhere near 30ppm.
Irrelevant? CO2 directly effects pH, so long as nothing else effecting pH (acids, buffers) are changing then CO2 is all thats left...
Again, if CO2 is the only thing changing you should be able to get a number that isnt totally corrupt or absolutely meaningless.
Case in point, the 1* pH drop method. Doesnt care what acids or buffers you have. If CO2 drops your tank pH by 1* than that is approximately 30ppm (assuming your water only holds ~3ppm naturally). Last I was aware this method was still valid and accurate, has that changed? I verified with my current 75G often by letting a sample of tank water degas for a few days and I get just about exactly a 1* rise (about 6.4 mid day to 7.4 after degas).
The water in a planted tank is not likely to contain only CO2 and carbonates as things that affect the pH and KH. The water likely does contain phosphates and tannins, and that means the relationship between pH, KH and ppm of CO2 is no longer valid. Even if you don't add phosphates to the water, tap water usually contains phosphates added to raise the pH above 7.
The 1.0 rise in pH method doesn't work because we have no easy way to determine how much CO2 is in the "degassed" water. If you do some experimenting you will find that the tank water pH will continue to rise for at least a couple of days when you let it sit out in the open air. Even after it stops rising, unless you find a way to measure the ppm of CO2 now in the water, you still don't know how much is present. All that the 1.0 rise in pH test tells you is that the ratio between ppm of CO2 in the tank and in a "degassed" sample of tank water is 10. Without knowing the degassed ppm of CO2 you can't find out from that what the tank ppm is. We used to assume that a degassed sample would have about 3 ppm of CO2 in it, but that is a bad assumption.
You can check this yourself, with some effort. Get some distilled water, add enough baking soda to get a KH of something like .5 dKH. Then use a straw to blow in the water and get some more ppm of CO2 dissolved in it. Now, use a pH probe to measure the pH of the water over a two day period. The equation relating ppm CO2 to pH and KH will be correct for that water, so you can calculate what the ppm of CO2 is at any time. If your results are similar to mine, that number will be as low as .5 ppm if you wait long enough.
Only the drop checker method gives "accurate" results, unless you spend a lot of money on laboratory equipment.