Yeah i would say its about time for a water change, imo
its easier to buy pre-mixed saltwater from your LFS
. that way its more than likely going to be the same every time.
i like to just do a 5gallon PWC
because it only takes 2-5gallon jugs, 1 for old water and 1 for new water. by all means do a 6gallon water change, whatever works for you, i just find it easier to only lug around 1 jug each week or so...
ive been in the SW
game only for 7months, and i dont think i changed my water for the first 2 or 3 months(rookie mistake) and now doing a 5g every 10 days, it really makes a difference in the overall health of your marine aquarium.
1 more thing, i would recommend cleaning your sand bed at least once every 2 months