It should never be said (or thought) that any method of CO2 use won't gas your animals. It has nothing to do with the method and everything to do with attention to detail and monitoring your tank. CO2 isn't something you setup as a one size fits all or, or a set it and forget. It requires tuning. On tank as small as you are mentioning, you need have as much control as possible, and probably not run it at night. I don't know if by DIY you are referencing yeast method or pressurized as some will call both DIY.
Anyway, bummer...accidents happen.