You don't have to use distilled water. I do because I have free access to it at work and I live in an area where the tap water is hard, high in calcium carbonate (which is not ideal in cooling systems involving aluminium). If you have 'soft' tap water then just use that. If your water has a high mineral content then you could just get 5 litres of deionised water from a car parts shop. It's up to you - depends how fussy you are really. What you don't want to do is spend money on something that ultimately you are going to drain away after a short time. Many people would just use whatever comes out of their tap regardless.

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