This morning I went to feed the fish in my 55 gal planted tank and about 1/3 of my fish appear to have died overnight! I lost my large angel, a red tailed shark, several plattys perhaps others. This is a mystery because I have done nothing new recently besides adjusting my CO2 injection through a higher pressure, Archaea ceramic diffuser - I had to adjust the low pressure on my Azoo CO2 regulator to get the bubbles flowing properly.
Yesterday, I changed 50% of the water as it was a no dosing day in my weekly EI fertilization dosing rotation. I did not change the canister filter as I had completely cleaned and changed that last week using only floss and the same ceramic bio rings. I did what I usually do as far as the water change, using the same buckets, same water from the kitchen faucet, same water conditioner and the tank temp reads about 82 degrees. I might have slightly overfilled the tank as the water is touching the glass tops in spots but I have done this before with no prior problems.
I don't think it was an oxygen issue because I have a 48" bubble wand buried in the substrate that runs from 11:00PM until 3:00 AM after the CO2 and lights go out to mix up some 02 with the CO2 left in the tank from injection and what the plants produce at night.
I was just telling my wife yesterday that I finally might have the right balance of light, CO2 and nutrients since my fish all look healthy and the plants looker better!
Any ideas or suggestions would be really appreciated.

Wow, that really sucks! Sorry to hear about so many deaths.

Since the regulator is the only thing you really messed with.....What is your output pressure now and what was it before? Did you adjust the bubble count to be the same? Do you have a drop checker?

Thanks for the quick response.

I adjusted the pressure from near 30 psi to about 35 psi and the tiny bubble flow from the ceramic diffuser seems better. I do not have a drop checker at this time and even though the needle valve on this regulator seems not to allow for super fine adjustments with this higher pressure diffuser, the bubble count is still at 2 bps. Between the CO2 flow, the canister filter output and my small circulation pump I believe that the CO2 is really getting evenly distributed to all the plants.

Amazingly, with whatever is happening some plattys, all of my corys, all of my rasporas, all of my Otos and my clown pleco appear to be fine.

First off, you need a drop checker. Without, you have no idea how much CO2 you're putting in the tank. I have read methods on the web that don't really make sense to me...the keep increasing until you see your fish start acting weird or gasping method - wrong way, IMO. High levels of CO2 will not kill right away, so that method is flawed from the start. CO2 levels can also kill, irregardless of how much O2 is in the tank also. That is to say you can't keep increasing CO2 as long as you increase O2 - just to be more clear.

The new diffuser you have is similar (different brand) to the ones I use in my tanks. I think they need a about 30psi to operate correctly. The thing to keep in mind about pressures vs bubbles per second is, if I increase my pressure and my bubble rate stays the same, my CO2 being diffused has also increased. Higher pressure actually produces a larger bubble, so that is more CO2. If you were pretty close on your CO2 levels and then increased, you could have pushed your tank to the point where it started affecting your fish.

I would back off slightly on your CO2. Get a drop checker and some drop checker 4dkh solution. I use these drop chekers Glass Drop Checker – Nano | The Shrimp Lab

I agree with Ben - drop checkers are cheap and are the best way to tell how your CO2 is doing - I used to use a pH-KH algorithm that was fairly accurate, except there were many different things in the tank that could screw up that algorithm.

__________________
Officially fishless until after the move....tune in later for details!

Thanks to both of you for your input - I do plan to purchase a drop checker, a different ceramic diffuser and a new bubble counter ASAP! I do not want to have this happen again.
I returned home this afternoon and realized that 3 Otos were also deceased so now I have only a couple of plattys and 6 H. Rasporas left alive - yikes!
Due to the Azoo CO2 Regulator that does not seem to want to play nice with the Archaea ceramic diffuser, I am going back to a limewood diffuser for now which allows me very precise control with the needle valve until my new ceramic diffuser arrives.
I will get this CO2 issue corrected, continue my EI dosing and then slowly restock the tank.

Thanks again to both of you.

I would stop dosing CO2 for a day and let your bubbler run for most of the day. Once you gas out most of the CO2, you can start up again. I would start at 1bps until the drop checker arrives. Just ensure you get the 4dkh solution or you will get erroneous readings with your drop checker. Green leaf aquariums sells it.

Hmmmm...this is a bit on the interesting side.

Have you ran a gambit of water tests by chance? How are you dosing EI?

Definately have to recommend that drop checker with a pressurized system.