Usually CO2 is counted in bubbles per second. You count the number of bubbles within one minute, then divide by 60 (approximating by counting 10 secs and multiplying by 6 will introduce a considerable error of measurement). Keep in mind this works only in pressurized system; using a needle valve in a
DIY yeast-sugar-baking soda system could be problematic.
To figure out the actual safe bps for a particular tank, you must first rule out
pH crash risk by:
a) making sure KH is within safe range for CO2 injection -aim at 6-8 range, adding a bit of sodium bicarbonate, or baking soda -be careful a tiny tiny bit will raise KH considerably-,
b) monitor fish behavior focusing on signs of CO2 poisoning, as gasping. The best time of the day to do so is at early morning, before you turn on your lights: at that time frame, tank's dissolved oxygen levels are supposed to be at their minimum. If your fish gasp for air, or stay too much time close to the surface, become lethargic or swim in odd ways.
Be ready, if you should, to cut back the injection of CO2 or consider using a timer/solenoid to control on-day/off-night cycles (for pressurized systems) or just remove power from the
powerhead (if using a DIY reactor) or simply let the CO2 go straight into the room but not into the tank at night time using a release valve.
Since you report a small tank (36x12x12) around 22 US gallons, aim at 2 bps and raise from there. I figure you should be very careful since your KH could be 2 or less and if you add too much CO2 (more than the plants can handle) the risk for a pH crash is big.
A CO2 drop checker, or a DIY version of it (see:
DIY CO2 monitor) would let you estimate the dissolved CO2
PPM in your tank, it's easy to do and safe for the fish. The advantage here is that pH in the drop checker (not the tank water) and constant KH will reasonably accurately translate into the KH/pH table for determining dissolved CO2. The error of measurement of a drop checker is small enough for our needs in fishkeeping, doing so straight from tank's water pH and KH will give you huge false readings.
Since you report elsewhere that you have an accurate pH reading instrument, you could still use it to adjust CO2 if you find a way to use that instrument in a DIY CO2 monitor (the simple principles of the CO2 drop checker allows creativity).
Nevertheless, for serious black water bio-type keeping, you should get a KH and
GH liquid reagent tests. Tetra,
API, Seachem... there are many available out there.
On the other hand, a DIY yeast-sugar-bicarbonate CO2 generator, with a efficient enough reactor should get you where yo need to without much risk. Begin with a 2L bottle and add another one only if needed. Keep in mind you cannot control CO2 flow directly.
Here's an easy way for that type of DIY efficient CO2 reactor that suits your tank very well, in my honest opinion:
DIY CO2 monitor
Hope this helps
Pepe
Santo Domingo
