topping off a tank

[Neophyte]

I have a general water in fishkeeping question. When the water evaporates in a tank and more tap water is used to top it off, doesn't the water become harder over time? I am thinking that the minerals don't evaporate, so shouldn't R/O or distilled be used for topping off?

My tap water is 8.4 and very hard so that is why I ask.

Also, would the water become more base over time?

[ronrca]

You are right! Hardness does not evaporate therefore by just topping your tanks off, the hardness will go up. Depends on how often you do your wc's and the volume!

[RandalB]

Neo,
IMO, if you need to top off water, you aren't doing enough water changes.

RandalB

[Carol_Roberts]

Randal is correct. You need to be doing daily or every other day water changes to reduce nitrAte, bacteria, fungus, and parasites swimming in the water. Your discus tanks should not get a chance to evaporate. I do 25+% water changes at least twice a week in my African tanks - they love clean water too.

[ronrca]

I only do 50% weekly wc's on my planted tank. There is evaporation by the time the next wc rolls around (about an inch lower than normal).

Generally speaking, it is better to top off with RO water or do a small water change to prevent hardness going up.