If you don't run a skimmer, an airstone is a good start. If you do run a protein skimmer, try running a tube outside for fresh air.
Point a powerhead towards the surface to get ripples, this will increase gas exchange and should decrease CO2 in the water. Likewise, if you are getting a film on the surface of the water, try breaking it up. Either powerheads or surface skimming.
If you have a reef, you can start using kalkwasser in your top-up water. This will increase your calcium and alk though. Definitely Google this method as there are drawbacks and dangers to avoid.
I would first get someone to double check your salinity and test your calcium and alkalinity. Your ph is low enough that this could be a contributing factor. This is also the easiest to fix!
Probably the most expensive but definitely effective method is to run a co2 scrubber. Guaranteed to work but costs a lot.
Don't make lots of sudden changes. Do one thin at a time and keep testing.