[JustOneMore20]

You can't really regulate DIY CO2 without the risk of building up pressure in the bottles, which isn't good. I assure you that 2 2L bottles on a 37g won't hurt the fish. I've had 2 2L bottles on a 3g setup with a Betta and while the bubbles were going at full force in the beginning, it eventually slows down. My Betta never showed any signs of too much CO2. I bet yours is producing alot of bubbles now because 1) the mix is new and 2) you used the highly reactive yeast.

If you are still worried, put an airstone in the tank attached to an airpump. The air will outgas some of the co2 and also increase the amount of oxygen in the tank.

Remember that co2 doesn't replace o2, so having alot of co2 in the tank won't make the oxygen less. Too much co2 can have negative effects on the fish and can look like there is too little oxygen (gasping at the surface) though.