70% is that really necessary??? Its a lot
I use water from my shower in n out with buckets lol
If a 70% water change is "necessary" then you've left it too long IMO, smaller and more frequent changes would be better, and if it isn't necessary you're just putting good water down the drain.
ps. go invest in some long lengths of hose before you ruin your back carrying all those buckets!! I haven't used a bucket for a WC in years.