I do not run CO2 injection but from reading post's from those who do,, I might consider placing CO2 on it's own timer so that it comes on an hour before light's do instead of placing it on same timer with light's.
And have C O2 go off one hour before light's go off for the day.
By starting CO2 an hour before light's come on,, Plant's will have ample CO2 from the outset and won't have to spend as much energy collecting CO2, but rather they can use most of their energy gathering light, nutrient's.
Some also say that plant's can use up most available CO2 in first four or five hours depending on ppm injected.
By end of photoperiod,the plant's have pretty much closed up shop for the day, so turning CO2 off an hour before light's are off will have no negative effect on plant's or fish.(save CO2)
I might also consider placing airstone on it's own timer, programmed to run at night while CO2 is off, and plant's are using available oxygen and fishes as well. This also would prevent CO2 from being driven off during the Day due to surface agitation.
Might also consider that frequent water changes may make dialing in CO2 difficult due to fluctuating level's from water changes,, and that being injected and or lost quickly during water change, or perhap's I am over thinking?
Might wonder when you say that fishes were Gasping before CO2 was introduced, if said fishes were gasping all the time ,or primarily at night or first light. If the latter was the case,, then the airstone at night should help but would not maybe be needed during the day.
If fishes gasping were primarily the Barb's, or Cory's, then perhaps temp's are too warm for these fishes ?(most prefer much cooler temp's)
Might also keep canister filter cleaned at least monthly by rinsing material regularly in tank water removed, to prevent biological or mechanical media from getting dirty and clogging or choking bacteria with dirt ,,and also to help provide proper flow for Even dispersement of CO2 throughout the tank.
Could maybe purchase small powerhead to use in place of airstones to gently create surface turbulence without breaking the surface?
This would avoid any measureable CO2 loss during the day,, and help provide oxygen exchange of evening.
Just some thought's that may or may not prove useful.