I didn't see a link so I googled the term. When I leaned of it, it was referred to me the inverted bottle method. Any way I do use it and it works very well. With this system drop checkers don't work which is not a problem since you cannot kill your fish with too much passive CO2. Drop checkers only detect the CO2 in the gas bubble between the indicator and tank water. When the drop checker is setup correctly it will only show good a good CO2 level when the CO2 level in the bubble is over 400ppm. So a drop checker show low CO2 when it is removed from the tank because atmospheric CO2 level at 400ppm is too low. So a aquarium with a drop checker has more CO2 than needed. and a lot of the CO2 bubbles from the diffuser never dissolve into the water. So a lot of CO2 is simply waisted.
In my 5 gallon tank with a diffuser I was refilling my20oz paintball bottle every 3 or 4 months and it wasn't easy to keep the flow stable. With passive CO2 I now have gone 18 months and it probably will have to replace the bottle in the next month or so (the regulator output pressures is getting a little unstable). In my system my bell is rather small and it needs refilling about 4 times a day. So I have set up my system on a timer that that opens the CO2 valve for 1 minute 4 times a day. I have not seen anything indicating a deficiency of CO2.
Note there a a lot of people that say if you are having problems growing your plants, you have a CO2 problem. Don't believe that. The most common issue growing plants is a nutrient deficiency. Using a fertilize is no guarantee that you don't have a deficiency and most commercial fertilizers don't have or don't have enough of all nutrients plants need. Most depend on your water having the nutrients Calcium, Magnesium, Copper, Zinc, and nickel. Most of the time it does, but sometimes it doesn't. A good way to tell if you have nutrient deficiency is to put a floating plant in the tank salvinia, or re root floaters. Floating plants get all the CO2 they need from the air. if the floating plant leaves look yellow, rot, fall off, or the plant simply doesn't grow you likely have a nutrient issue and not a CO2 issue.
