[Phil Edwards]

I live in AZ (US) and the tap water there is very hard. Tds was above 800. Of course I don't know what's in there, probably Ca and Mg mostly. I was scared so I used half RO half tap for my first tank. I'm getting tired of RO now, so I'm thinking about using tap, but not sure if this will affect fish and plants. My fish are mostly from South America. Plants are java fern, sprite, crypt, anubias, amz sword,...

Depending on where you're getting your city water from it's not unexpected that GH and KH would be that high. A lot of rivers in that region flow through old ocean beds and pick up a lot of minerals that way and via ground water inputs. In general, as long as you're good about acclimating the tank slowly to your tap, in your case it should be just fine. All the plants you have in there appreciate a fair amount of hardness and alkalinity; especially if your CO2's on the low/non-existent side. Amazon swords in particular are Ca hogs. I kept Discus and many different kinds of plants in Dallas tap water (liquid concrete) for years with no issue except for the really sensitive plants and I blame that on other factors. The species you have there will be a-ok.

Regards,
Phil