Although it is common to use tap water, I recommend only using purified water for reef tanks!
Tap water has several disadvantages:
- it might contain unwanted nutritions (Nitrates, Phosphates)
- it might have harmful substances within (iron, copper, lead, …)
- Trace elements like Ca/Alk/Mg are already in the water. Mixing water for water change with salt mixtures will ADD this trace elements again and thus be “to high”
- Variations in water quality are typically between seasons
0. At winter most water works add silicates
0. At summer often chlorides are added
- Sedimentation of your houses piping might reach your tank
For most tanks a reverse osmosis unit (RO) with additional water purification (silicate filter) is a good choice
- RO: removes nearly all unwanted material on a mechanical way
- Silicate filter removes silicate still remaining after RO as well as further substances on a chemical way
There are more options of water purification that might be interesting for big tanks.
For small/Nano tanks usage of distilled water is also a good option.