You can use RO water to lower pH and KH. Some people mix RO water and tap. Others use 100% RO water but they have to remineralize before poring in tanks. LFS (not chain ones but an individually owned fish dedicated store) should sell RO water.
I've never used RO, so I can't explain details. You can google more for details or talk to employee at LFS (again, not chain ones).
Tetras and loaches usually prefer pH on the low side. There are some fish that prefer higher pH. I guess you will be able to have better luck with them.