Water quality as in nature is highly important for the animals as well as the plants in an aquascape. Tap water has been known to contain problematic substances like heavy metals (e.g. copper) in too large a quantity, or maybe treated with chlorine. Fish and shrimps will not do well in such water, and many aquatic plants will grow poorly under these conditions.
Dechlor 500ml is a chlorine neutralizer for removing free chlorine in tap water.