I would not suggest using any PH down product, most are just phosphates and will only bring about new problems. I suggest mixing RO with tap water to get your KH down to around 3, this will yeald a nice PH for cardinals and still keep PH from fluctuating too much with the addition of CO2. While CO2 alone can be used to lower PH, you can't... or shouldn't exceed 30ppm of CO2, so there's always a limit to just how low you can go before you have no other choice other than to reduce KH.
Further, it is my opinion that most of these fish are not actually triggered by the drop in PH, rather the drop in hardness or KH (potentially also GH and TDS). While a drop in KH does bring along a drop in PH, a drop in PH caused by an increase in CO2 will not alter KH. Also, unlike in reef tanks, I doubt you will be able to rely on plants to soften the water, you would need to not do water changes for way too long, bringing about problems of accumulation of nitrates and other organic substances.
Now if RO or other sources of distilled or natuarally softer waters such as natural springs are out of the question... I'd say leave the KH and PH where it is, you are not stressing the cardinals in any way, they will adapt better to a higher yet stable PH than a fluctuating one which would be likely if softening chemicals are used. The only thing you may want to try is adding some peat in a filter bag in your filter, this will reduce the PH for you without the problems phosphate buffers can bring about. Your plants by the way will likely be uneffected by the change in KH and/or PH. Just because some plants come from softer waters doesn't mean they do better there, they may have the competitive advantage of being able to cope with an inferior source of carbonates but nearly all will do just as well in harder waters.
Hope that helps
Giancarlo
