Should I increase the canister filter flow rate to increase surface agitation. I am worried that there’s not enough oxygen for the shrimps but at the same time, I don’t want to lose CO2 from too much surface agitation.
I’m going to address your question on O2 levels vis-à-vis surface agitation. Your [WOW!] fish load, I’ll leave to others.
First, the solubility limit of oxygen (presuming an average atmospheric pressure in NA) in water that is 78 degrees F is about 8ppm (colder water has higher solubility). Incidentally, this is much lower than the solubility of CO2 and, fortunately, they are independent of each other (CO2 doesn’t displace O2 and vice versa).
Although it is hard to compare expectations for O2 levels between tanks, because there are so many variables that can affect it, particularly varying degrees of bio-demand by flora (which includes bacteria) and fauna, below is my experience with the Salifert O2 test kit, also mentioned by @mboley
. My 29-gal has a surface area that is about 7% of the total volume, which affects the speed of gas exchange. My canister filter return is completely underwater, so the only agitation is created by a small surface skimmer, which makes it easy to test gas exchange. I have a very heavy fish load and a large, healthy plant mass.
I do not know how to get at a good calibration for oxygen tests without spending a lot of money, so I use it as reference indicator to other readings on this one tank, similar to how a TDS meter would be used. My Salifert kit reads 12ppm for degassed water. I take this to mean that a Salifert 12ppm indicated level is actually 8ppm (maximum saturation). When I run the skimmer, I reach the 8ppm (Salifert 12ppm) level in my tank and it holds day and night, despite significant O2 changes due to photosynthesis cycles. Thus, it proves, to me, that surface agitation makes a big difference. I would suggest that you could use the 12ppm Salifert level as a guide to whether or not you are achieving full - normal (see supersaturation below) - O2 saturation at 78 degrees F. You could also find your own solubility level if your water temperature and/or atmospheric pressure is significantly different (there are charts on the web).
When I shut my skimmer off, during the day at maximum photosynthesis (it would be different in the dark), The Salifert reading drops to 8ppm after about 4 hours, despite the heavy photosynthesis (which can add more O2 than the gas exchange process). I assume that the Salifert 8ppm means an absolute change from the 12ppm vs. a relative change (if any members can confirm, I’d appreciate it), so I take it as an actual ppm level of about 4ppm (8ppm actual - 4ppm actual) and I see my fish struggling at the surface. My fish load clearly consumes large quantities (relative to saturation levels) of oxygen. When I turn the skimmer back on, O2 returns to the Salifert 12ppm and fish behavior returns to normal.
Believe it or not, too much oxygen (supersaturation) can stress fish (gas bubble disease, where you can see tiny bubbles on fish. It’s like the “bends” in humans), but this is hard to do. O2 levels can be pushed past the normal solubility limit with quick temperature increases or very rapid photosynthesis with lots of plants and/or a sudden algae bloom. I wonder if a sudden lights-on at very high PAR levels and high CO2 levels would do this. I don’t recall seeing any complaints from members, but I’ve seen the symptoms mentioned without any suggestion that GBD may be behind it.