"Liquid things that lowers pH" is not a good thing to use ... you get yo-yo-ing pH more often than not. <There are chemical means to remove carbonates from water, but that is a different thing altogether.>
The best way to deal with water that is heavily contaminated is to use a reverse osmosis unit to strip out all the ions in the water. Then you have to add back minerals to the R/O water to maintain buffering capacity (& hence stable pH) & for minerals needed for fish (or plants or inverts.) Some people just mix in tap water for this (assuming that the tap is just hard & not toxic). But if there are nasties in the tap, it is best to use a comercial salt mix (different kinds for different fish requirements --- ie low pH vs high pH)