I really was just trying it out today as I just got it ordered from Drs. Foster and Smith, so I did not vacuum substrate today, as it would have been easier. But when I do the gravel vac's seperately it keeps the tank clearer becasue essentialy I am taking some tank water out and replacing it with new water so it is a little cleaner, lol like I said I am a stickler for water parameters and keeping them at 0 when they go up from 0 I do at least a 30%-50% WC and sometimes vac gravel at the same time. I did this mainly becasue of the PITA it was to siphon into 5 gallon bucket stop siphon, empty bucket, restart siphon refill bucket, dump again....now that it is so easy to do WC's and gravel vac I'll prob. start doing them all at once. It just kinda burns you out changing water on 3 tanks in one day and then having to vac grav all of them, and I am one to complain with only a 90, 55, and a 29, (have a 75 on lay-a-way) when ppl on here with 3 240's have to do 50% wc's on them every week lol.