ok... why has nobody mentioned that pH swings due to co2 are not dangerous to fish like a crash from lack of buffer?
also... 10ppm co2 is not overdoing it in the least. you pH may have crashed, but it was due to the no buffer situ, not the co2.
Simple, CO2 is not a "salt". It does not affect osmoregulation like KH does.
There are many many references about pH, but they do address additions of CO2 gas's influence on pH, so folks think it's pH change that causes issues and they apply this broadly for all cases, even where CO2 is added, and thus a myth is "born".
"Large pH swings from adding CO2 is bad for fish."
This has no basis and succinctly put: a myth.
There is
physiological support,
observational support(plant folks have been doing it for decades without so much as an issue with the most sensitive species),
natural ecosystem support where large plant biomass changes the CO2 content every day , often moving the pH by 2-3 units, in some places, as much as 4 pH units every day and where fish are present.
KH however stays relatively stable.
pH changes even if you have no to very little KH due to CO2 also have no known negative effects, at least I've never been able to find any issues with super soft KH valves at 10ppm or less of alkalinity.
Plants want CO2.
I cannot directly measure CO2 as you get below about 10ppm of KH, but the CO2 dosing rate and concentration does not change if all you change is the KH nor does the plant demand.
If the KH keeps dropping, then it likely means you do not have enough CO2 in the system for the light and plant biomass you have. Thus adding more CO2 and adding a bit of baking soda is useful.
Plants will use the KH as a carbon source if the CO2 ppm are limiting.
So that is a good sign you are not adding enough CO2.
13$ ought to be enough to DIY a good CO2 reactor, but you still need good flow in the tank also.
Regards,
Tom Barr