Does low co2 cause less evaporation? My tank always evaporates 1 gallon a day. We went on vacation for 4 days and came home and my tank only evaporated 2 gallons instead of 4. Also my ph was 2.6 and is usually 1.6
PH of 2.6 and 1.6 can not be the right numbers - it should be around 8 or so. But whatever the numbers actually are; when your PH goes up, your CO2 goes down. But those shifts won't generally cause evaporation to change.
Do you often keep any doors and windows open that were closed while you were away? That may have changed the relative humidity of the room, which could impact evaporation rates.
Did you change the settings of your HVAC system while you were away (or do anything else that could have impacted the temperature of the tank)?