20 gal tank - good mix of community fish ?

If you have RO unit at home, then you can use 100% RO water.    The problem with RO water is almost all of its minerals (water buffer) had been stripped.   Any slight change can swing its PH by huge amount.  What you need is to add the mineral back to restore its water buffer.   The product you need is Seachem equilibrium.   Be very careful on the instruction.  It is still a lot simpler and safer than trying to mess with tap water.