Depends.
my view was that more bacteria and fish meant more respiration and therefore more co2... am I barking up the wrong tree?
When you step towards specifics rather than principles the knowledge can quickly take you away from usable, executable solutions. Not to throw the baby out with the bath water.
Yes livestock will produce a byproduct of Co2. How much? They also produce waste that uses up oxygen when being processed by bacteria. Is it usable in the relationship of Co2 to oxygen through photosynthesis by plants, is it consistent?
What relationship do you see between water temperature, pH, oxygen, nutrition and toxins for ‘bacteria’? More of what specific groups of bacteria? What is their rate of division? Even cleaning your filter sponge of decaying organics will cause die off of large amounts of the heterotrophic bacteria that were dependant on organics to maintain their colony. This die off then uses up more oxygen. Is there enough to cope or has your water gone cloudy?
I’m stepping off the track of solely using biological means of getting oxygen (as a product of photosynthesis in low tech) into water and championing the idea that using gas exchange is a more stable and reliable means of achieving this. It’s consistent and isn’t as dependant on so many variables.
What is of relevance here is the scale of the answer you’re looking for. If it’s restricted to the amount of Co2 produced biologically then fine. If it’s in how the amount of Co2 relates to oxygen saturation in water through biological means in the system then cool. If it’s about how to achieve a balance in an aquarium system so it’s relatively stable over a 24 hour period so everything is dependant on a key variable of oxygen, then we’re at a system level of thinking.
A separate arc to this is gas exchange is not exclusive to oxygen obviously, it also leads to consistent levels of Co2 based on atmospheric levels. But Co2 is important during the photo period, oxygen gets no rest. It’s required 24/7.
High tech systems are a different animal yet again that require a different design to low tech. An obvious statement but these days I think less in terms of ‘high tech versus low tech’ and more in terms of how hard you’re pushing that accelerator peddle down, and also what does this affect within the limitations of the aquarium system?