Can I just top up with tapwater?
Nope. Tapwater contains chlorine (and possibly chloramine, depending on where you live) and is toxic to fish, and to the vital bacteria living in the filter. Before adding any new water to the tank you must treat it with a special additive called a water conditioner or dechlorinator. All shops sell these products, and they only cost a few pounds for a bottle, so it’s not particularly expensive to use. When you top up, just put the recommended amount of conditioner into the bucket of tapwater, give it a swirl around and then slowly pour it into the aquarium.
