I used water "stop" for years but from time to time I got some staaining on prints that wasn't visible for some time. My fixer (two bath in a Nova slot processor) was always fresh. Now I use an acid stop bath, not acetic but sodium metabisulphite which I think is more compatible with neutral fixer. It is very smelly (SO2) but in the slot it's not as bad as an open tray. I have not had a stain since doing this.