Gauging CO2 stress levels in fish

I'm in week 2 of a new planted tank setup, and I'm in the process of coming to understand the flow patterns of my tank, getting the CO2 dialed in, etc. I've been gradually tweaking the CO2 levels up bit by bit, keeping an eye on the tank inhabitants as I go.

I've noticed that of late the Otos (of which there are about 15 now) have been tending to hang out in the back left corner of the tank, which is also where there happens to be the least amount of flow. Not all of them stay there; you see a couple out and about in other areas. But they mostly hang out there.

They don't seem unhappy per se - they keep active, and they're not gasping at the surface or anything. But them hanging out in that limited area does make me wonder if CO2 levels are so high that that corner of the tank is the only area where they can remain in their comfort zone.

Does that sound plausible? Or is it usually more obvious when CO2 levels are too high? Does anyone have any thoughts, general or specific, about how one develops a "feel" for this kind of thing?

[FWIW, the tank is 77 gallons, and the bubble count is just under 6bps. CO2 is being distributed by an ADA pollen glass.]

Tom Barr;37014 said:

Larger and more active species will be more susceptible in general.

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That's interesting - I hadn't heard that before. It'll be good to bear that in mind later on when I move a couple of good-sized angelfish from another tank over to this one. Worth dialing the CO2 back a notch or two when I first introduce them, then bring it slowly back up?

I'm backing off the CO2 a wee bit today to see if the Otos retreat back to the corner again at the end of the day. If they don't, I guess that could be an indication that I was riding the edge a little bit yesterday.

Yeah, it sounds like gauging CO2 stress is another one of those things that you just have to get a feel for over time. Your tips are a helpful starting point though, Tom - thanks.

This is interesting, subtle stuff. Starting a planted tank has definitely reignited my interested in aquaria, there's so much more to consider now.

Philosophos;37027 said:

My experience with otocinclus is that they hide most of the time until they're comfortable with their surroundings; corners are favorite places. Some never seem to come out during the day. Gassed oto's have always just sat on the bottom for me, gasping with little motion.

-Philosophos

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Also good to know, thanks! I noticed yesterday that after the tank lights went out, the otos were out in force. It was difficult to tell, though, if that was because of it being "night" for them, or because the CO2 was turned off by then. I don't really have much experience with otos, so for me this is all new.

I probably won't worry too much more about it until I start putting in the cardinal and rummynose tetras. Maybe their behavior will be easier to interpret.

Cheers,

~Jeremy