You should only top up with coolant, not water and not Tap water either!!!!! use ready mixed coolant as it is easier. I would suggest if you have been using just water for topping up for 18 months, then drain the lot out and refill with ready mixed OAT coolant (pinky orange colour) you will need 10 litres to refill the system and make sure you bleed at all three bleed points, 1. radiator, 2. heater matrix and 3. the cooling rail on the engine. make sure heater is set to full hot before draining and refilling.
good plan to also replace the cap on the expansion bottle, with an OEM one not an ebay cheapie!!