According to the kh/pH table, I need to hit 6.5ph to get optimal co2.
Running at 2bps the pH does drop to 6.5 but only after leaving co2 on for 6 hours, an hour prior to it turning off.
In my experience your PH of 6.5 with CO2 is more than sufficient for your plants.
The PH KH tables are based on a lab sample of sodium bicarbonate and pure water with various levels of CO2 in it.
It is an extremely rough guide and is not in any way accurate for estimating CO2 levels in an aquarium with different types KH compounds and the minerals in the tank. In fact a drop checker doesn't measure the CO2 levels in the tank water. it simply tells you your tank is outgassing CO2. And for outgassing of CO2 to occur, CO2 levels in the water must exceed the atmospheric concentration of air ( about 400ppm). If you take a drop checker out of the tank and set it on a table it will eventually tell you CO2 levels are low. But if you put it in a bottle full of CO2 gas with no water it willl tell you you have high CO2 levels.
In my tank I use the
inverted bottle methode of getting CO2 into my tank. It is extremely efficient and my CO2 bottles last a lot longer than they did with a diffuser. With a diffuser I was refilling my CO2 bottle about once every 3 months. With the inverted bottle methode i can get about 1 year on the same bottle. I don't have a drop checker because there is simply no way for you to get CO2 concentrations high enough to harm animals or for a drop checker to work. My co2 is on 24 hours a day and a time automatically refills the small 150ml bottle I have in my about 4 times a day. in the morning my PH is about 6.8 when the lights turn on. When lights turn off it is about 7.2. Note ten close to a 1 point drop.
I don't know where Tom Barr got the data from for his chart and don't understand how the CO2 is meant to increase as the pH decreases so I won't comment on that.
When tom Barr published his Estimative index fertilization methode for CO2 he said that if all plant nutrients were present any plant growth issuses would be caused by low CO2. Many people that have had success using his methode use a PH drop of 1 point as a rough guide and then adjust the CO2 as need to get the color right in a drop checker. Many users have used the 1 point ph drop guideline with fish in the tank and CO2 off at night with no long term problems observed.
It is now known that Tom's statement about CO2 is incorrect because Tom ignored the nutrients his tap water provided (Calcium, Magnesium, sulfur chloride, zinc). His widely copied fertilzer recipe in my experience doesn't work with RO water.
since his recipe doesn't have calcium, magnesium, and sulfur in it and ignored Chlorides and he assumed his fertilizer had enough zinc (CSM+B has about 1/20th of the zinc needed). But his water corrected his errors. So many people blamed CO2 for there problems and went nuts trying to correct it. So some people like myself has stopped using CSM+B and other rpurchased fertilizers. Instead Purchase the ingredients separately and started to make your own micro nutrient fertilizer. And then instead of just dosing just NPK like TOM did we dose the macros (N,K,Ca,Mg,P,S and chloride as needed.
Note chloride is a chlorine containing salt such as calcium chloride and sodium chloride. Chloride salts are safe for plant but chlorine gas will kill them. All living things need some chlorine in the form of chloride salt. Tap water is typically rich in chlorides due to the water treatment process.
