What is the tap's KH and GH? I'm guessing it's undoubtedly harder than or holds the same hardness that the cichlid substrate does. That's probably why you're seeing no difference in the tank full of wood and regular Eco. Not to mention there are the bad batches of Eco going around, and a temporary effect from good Eco as Imeridian mentioned. If super hard tap water is the case then you didn't even need the cichlid substrate to achieve more hardness (which is the important factor for hard water species over a specific pH, and vice versa for soft water fish).
Many things can alter pH without affecting hardness and the fish don't care. More important to match their natural hardness and ignore pH, it will follow hardness. Just remember that you're concerned with GH more than KH when you're matching a fish's natural hardness, lowering GH by dilution for soft water species will automatically lower KH (buffering capacity) and pH will follow. This is where people get hung up on what pH a species supposedly requires and most profiles don't clarify desired hardness unless it's somewhere like fishbase. If you mix it down to say 3dKH, you will probably be looking at a pH of around 7.2, GH will most likely be in the 'soft' category at that point, depending on the Ca and Mg levels etc in your tap. If GH is still too high at that point then you can mix further down, then if you choose to have some buffering capacity present you can add a bit of baking soda to bring KH up but the water will still be considered soft due to the low GH target, just be aware that bumping the KH will bump the pH, but GH is what you should be concerned with, not pH.
Too many people use store bought snake oils or some source of acid in their discus tanks etc and don't even realize that their water is still hard because they never test GH, they only concentrate on pH and probably don't even know what their KH is either.
I agree an RO mix is your best bet to maintain an easy, constant, steady target. A $60-100 4 to 6 stage RO unit from ebay will get the job done and is cost effective over the long haul, especially if the tank is large or there are multiple tanks. Lugging bottles from the store every week gets old fast.
If you do use store water, make sure it has no minerals added. The refill stations out front usually list the filtration process on the front which often includes RO and maybe DI or UV, usually carbon block also. The commercial bottles will list whether there is any calcium, magnesium or potassium added, you don't want that.