CO2 usage - fish seem to be gasping at the surface quite often

HI Guys,
I have my 240L which has been set up for a few months now (not my first tank by a long shot, but it is my first CO2 tank)
The setup is - Lights and CO2 are on timers, the CO2 comes on a half hour before the lights, and shuts off about an hour before the lights start to ramp down in the evening.
Question is, my fish seem to be gasping at the surface quite often, not always, but a good 3-4 fish at any time are gasping at the surface. Obviously this is due to lack of oxygen, but how do I both oxygenate and diffuse CO2?

 

Do you have a bubble checker on your co2 system?

 

Do you have a bubble checker on your co2 system?

I don’t, I’ve ordered the drop checker and liquid on amazon, but with this whole Coronavirus thing it’s yet to arrive.
I’m at about 1.5 bubbles per second though

 

I don't use co2 so I'm not sure how to estimate it without the drop checker. Its a balancing act so the only thing I can suggest just going by the actions of the fish is to try 1 bubble per second till you get the drop checker.

 

But that only tells me the level of CO2 in the water, not the oxygen? Am I right in thinking it measures between low levels of CO2, normal, and lethal?

 

Yes it only measures how much co2 is in the water. And yes it indicates low, normal and lethal. So your 1.5 bubbles may be in the high normal or low lethal range. I'm only going on the action of the fish with that theory though.