If you had a bare bottom or didn’t disturb anything (not likely) you’ll want to use new. I did run into the issue of having no nutrients in there (it was low to begin with) so if you have very low nutrients as is, you’ll want to carry over some of the water or dose nitrate and phosphate. If you can do a significant water change and still have over 5-10 nitrates then you can use mostly new water.
All systems are different, I had to dose for months because I started with mostly new water and I had <5ish nitrates to start.
In case you need more things to worry about lol