First check the KH. That is how to avoid PH crashes. KH needs to be 3 degrees or higher, or the added CO2 can drive the PH much too low. If the KH is below 3, add ordinary bicarbonate of soda (baking soda) to raise the KH. With DIY CO2 the risk of getting too much CO2 is far less than with pressurized CO2, where end of tank dumps are possible.