Passive CO2 bell depletion questions

[Corbidorbidoodle]

I added a passive CO2 bell to several of my tanks. I saw the one on the big display tank at the CoOp and figured I could give it a try.

Seems to be doing some good. I haven't tested for CO2 directly, but I'm seeing improved growth, and the Ph has dropped measurably.

Here's my question though. I've noticed that on every tank, the first day the water soaks up almost the whole bell's worth of gas. Then the next day, somewhat less is depleted. And after several days, it only absorbs a little bit. This seems to be the case from my 5 gal to my 55 gal.

Is this normal? Does it mean the water is saturated with as much CO2 as it can from a passive system? Does it mean that most of the gas in the bell is just atmosphere?

And this is sort of another question, but I'm not sure how answerable it is by the average nerm. Is the CO2 absorbed related to how much the plants are using? Or more just about now much the water will suck out? So, like, if the plants are using a lot, will the bell deplete faster? Or is it not really related to plant absorption at all?

Thanks, everybody!

[Seattle_Aquarist]

Hi @Corbidorbidoodle

Yes, that would be normal.  Initially the tank CO2 is at equilibrium based upon fish, plants, and the CO2 level in the air.  When you first add the bell the pressure inside the bell pushes the CO2 into the water column and depending upon the size of the tank it could use up all or almost all the CO2 that was added.  The next day the depending upon surface agitation (the more surface agitation the greater the CO2 loss) the CO2 level is already somewhat elevated so not as much CO2 is needed for the tank to reach equilibrium.  Now, as the plants grow they will use more CO2 and fertilizer and the amount of biomass increases. Plants need light and nutrients and CO2 is considered a nutrient. -Roy