If you use an electric gravel vac I'd suggest testing for Ammonia, nitrite & nitrate before and after just to see if there is any noticeable change.
My 75G is heavily planted and sits at 10ppm nitrate if I leave it alone.. I change the water perhaps monthly and it stays crystal clear! My 30G with one goldfish tends to need weekly (or every other) changes.
I just test the nitrates to determine when a water change is needed. If it's much over 10ppm I'll do a change to bring it down to 10 or under, here's how I determine it...
20ppm - 50%
30ppm - 66% (2/3)
If you're much over 40ppm do a second change in a couple days.