hard to tell without pics, and more info (such as amount of LR, sand, skimmer, etc) but I would guess that you have less than 60g actual water volume...so 10g every 2 weeks would be doing good!i have a 57 gallon with a 20 gallon sump and i do 10 gallon water changes.
should i do a water change every week or every 2 weeks?