Which of my fish Are the most sensitive to CO2

Hi I'm currently running CO2 on my 60l for the first time fish inside are
5corydoras (mixed)
10 Neons
5 mountain minnows
1 betta
and 1 rainbow shark
I had 6 corys and 1 mysteriously died after looking sick for a few days I was wondering which of my fish would show the first signs of dangerous co2 level I do have an indicator in my tank but as we know it takes time for it to change colour

 

@carl leathwhite

My knowledge of Co2 is limited so im not sure if your cory died from the Co2 levels however, i do know that the rainbow sharks get quite nasty and can very well be what killed your cory so keep an eye there

 

Corydoras have a secondary breathing mechanism so elevated co2 levels would probably affect other fish before the corydoras. The classic sign of co2 overdosing is fish gasping for air

 

Any recommendations I think I may move the rainbow shark into my larger 270L whould that make a difference