I was told distilled water, but iv never had to, am a bit of a hoarder and keep excess stock of almost everything, 3 bottles of each peroxide strength and when one is used its replaced so there I always 3, loo roll's are always back stocked, sugar, long life milk for emergency, at least 2 cans and packets of everything in. Best to just go and buy a spare bottle of each ad keep replacing when you are on the second bottle, that way you won't ever have to dilute again unless putting the colour through the ends with the left over tint xoxo