i can tell you how i lowered my 8ph tap water to 6-5.5 , if you like. the first thing to check is your substrate, some will buffer your ph and never let it go. get sand that will not alter or specific substrates that will not alter ph or better even lower it a little. you also have to mob the tank with driftwood, I have about 5 pieces in my 30g. I also filter with peat by putting sera peat balls in a pair of womens tights and putting it with the filter sponge (this is the least effective bit because i never change them, too expensive). I also use diy co2 which helps destabalise the water buffers allowing the bogwood etc to bring down the ph further, it also lowers the ph itself. last but not least 4-5 cattalpa leaves help a fair bit. Now two things to look out for, first buy an electric ph meter, you need to check a lot, so liquid testers are a waste of money. second you will need a ph down for water changes. you can alternatively use co2 tabs to bring the ph down in the bucket before adding. remember the ph meter is essential kit (£10 e-bay) for this weekly process. but after you succeed, you can include all those lovely apistogramma species that would usually fail= well worth it. I warn you I am no expert but my method does not harm the fish because i keep fairly sensitive apisto's with no problems (they even breed in the tank). RO water i imagine is a good investment but only for a fish room not one tank.