I have 2 lowtech rainwater tanks, and 1 tapwater. The biggest is 65l, and I mostly do a 30% change weekly. I just use a siphon + bucket to take out a buckets worth, about 20L from my bigger tank. Before that, I will have filled a 30L carton of water at my allotment or back garden (water from allotment is more crystal clear, but also more annoying), then lug it up the stairs (the tanks are on the 2nd floor hooray), pour it into a watering can and fill it up again. During winter I've started collecting the water much earlier so it can warm up in my bathroom for a few days.
It is a little annoying, but it's made me a bit fitter and since my tanks aren't too big it's not too much bother. I prefer using the rainwater and knowing my fish will live better in it that my very hard tap. I dream of having a big tank with a proper waterchanging system one day built into the house one day though!