As far as i know EI regime does nothing with KH nor PH.. Most peopel use or Acid from a bottle, phosporic acid, vinigar can be use, some people use Oak leaf extract, Kapata leaf extract to lower the PH. But it doesn't hold very long, maybe a few hours and you'll see it rise again.. So you have to keep adding a dose over a daily schedule. The risk is the buffering capacity, you maybe start with adding 20 ml to lower it 1 ph unit and the next unit only need a few drops instead of ml. You are risking a PH crash and you do not see it comming slowly, you see it suddenly dive to ph 5 or 4 and then you have to do an emidiate waterchange or use PH+ again. And this is what you do not want in the tank, the fish and plants in there even less. The lower the KH is the sooner it will crash.
Other more steady and natural ways to lower PH is filter over peat, or use alder cones, or kapata leaves and or other leaves like oak, wallnut, beach but you would need rather a lot of this stuff and it also will stain your water tea colored. And they do not work for ever, this also you would need to keep adding and replaced each few weeks.
For a biotope setup this stained water can be what you want, it's a matter of preference, if you don't mind that throw a bag of peat in your filter and replece it every few weeks..
For a normal community tank, this is all to much hassle and it will not bring you any drastic extra benefits. I'm not even so sure about it if it will be cheaper as investing in a little co2 setup.. Since you have to keep buying liquid carbon, peat and whatever else you want to throw in.. Adding co2 will need a bit more investment to start it up, but in the long run it isn't more expensive, after you bought all the hardware needed you only need to buy co2. I run my 54 liter tank over a year with €25 co2. Every day 2ml liquid carbon and every 6 weeks a new bag of peat will cost me more and has less effect.
