When you say they hang out near the top, do you mean going right up to the surface? Or hanging out in the upper third of the tank? Big difference. Most fish have a preferred area to hang out in....lower third, middle, or top. Not sure with the fish you have, but it's possible that's just the natural behavior.
In general, a drop from 6.8 to 5.8 should not have any affect on the fish. Unless of course you are low on oxygen. Do you have any surface agitation? CO2 and O2 are not mutually exclusive and you want high levels of each.
The other possibility is that your degassed value is off for some reason. Are you dosing anything to add KH? If so, how much and what is the KH?
I would say that the fish that are near the top are in the top 1/3 of the tank, but many of them are still all over the bottom of the tank as well. It is not all of the fish, just some of them.
The only surface agitation I have (such as it is) is my canister filter outputs I have pointed slightly up so that I get some surface agitation, but other than my airstones that come on after lights out, that is all that I am doing.
I tested the degassed value of my RODI water as I think you suggested. But @mboley
suggested that maybe I was supposed to be checking the degassed value of my tank water instead? I checked the pH before dosing with Equilibrium to 3dGH, I am doing nothing at all for gKH. I suppose the pH could be off as a result, but I am using a fully calibrated pH probe to test, so I don't really think it was off unless I should have tested after
I dosed for GH.
In doing more reading, it says that if they do not have enough O2 that they would "gasp for air at the surface" or "hang out near filter outputs" and I do not see any of this at all. I think I might just be paranoid!
I just did a water change two days ago so I will do my measurements and see where we are at.
It makes more sense to me to degass your actual tank water because that's the water your fish are habitating. Your Ph in the tank
could be different than your reconstituted RO water. I'm going to do that comparison myself.
I will do this as well and see what my numbers look like pH wise after degassing some tank water...