You're not going to see an immediate benefit. Just like the tapwater usage has taken time to show possible symptoms, the phosphates and such that could be in there aren't going to just disappear because you start using store bought water. This will take time. Your tank has the potential to get worse before it gets better. Do regular water changes, put a fish in there...I kept a single chromis in mine for a long time. Feed very lightly. Keep the flow up. Have a good cleanup crew.
To be honest, heck, if it were me I'd probably start it over. I started my 10g over about 3 or 4 times in about 4 years. For different reasons, but I did it. That's another thing that's so cool about these nanos, ease of redoing things if the mood hits you. This last time I did it, I put a closed loop on it and love it much more than having a powerhead in that little tank. Unfortunately, I have not had the time to take care of it properly of late and, after years of having fun with it, I'm going to be taking it down soon, only to be used for coral quarantining in the future.