There's absolutely no need to buy bottled water in Slovakia, the tap water is safe to drink – good news for wallets and for the environment. Other than instances where a crossed-out warning symbol is posted above a tap (for example, in some public toilets) you can expect water issuing from taps to be potable. Travellers prone to upset stomachs may choose to bring a portable water filter for peace of mind when drinking tap water.
Note that mineral-rich waters are highly prized in Slovakia, and in spa towns you are likely to see locals queuing to fill containers with water piped from underground sources. Because of the mineral content, this water can be warm, fizzy or with a strong metallic taste. Travellers unused to it may find that these mineral waters cause nausea. Limit yourself to a few sips if you have a delicate stomach.