Ph changes from co2 don't really matter to fish if you were worried about your tank not being soft water any more. I have not found excel to lower my ph at all. I just measured the ph of Excel with my meter and it came out about 5.5 so its possible if you add enough but it would probably kill your fish from excel OD before ph change.
Excel is an organic source of carbon. It's not comparative to CO2 and will not lower the pH. I know a lot of folks know the benefits of CO2 gas, and I can't stress enough the importance of a pressurized system. Of the people I've convinced to drop the $150-$200 on it, none have regretted it. Heck, you don't even have to have fancy $$$ lights to reap the benefits.
If you can you should keep using the DIY at least. Excel is good if you have nothing or in combination with DIY. If you do some searching you can find posts about why co2 ph change doesn't matter to fish.
ER9: Why do you want to lower your pH? Unless you are trying to breed a very softwater fish (like discus), it really shouldn't matter. If you really have your heart set on lowering your pH you can add some RO water.