Rate of change is the same, regardless of the size of the tank.
A quarter change on a weekly basis should be sufficient. Don't over-feed the fish, or try to house too many. Check the ammonia level and pH regularly. Make sure the dissloved oxygen level is spot on too; low ammonia and average pH usually means D.O. is good. As a rough guide, 100% oxygen saturation at 760 mm Mercury or 1 Bar 1000 millibars atmospheric pressure and sea level gives a Dissolved oxygen (D.O.) of 9.3 mg/l - most species can handle 6 mg/l D.O.
Imagine being stuck in an air-filled room with several other people - you all breathe at different rates, so the oxygen level will change in favour of carbon dioxide, and you will all eventually perish. Translate this to a tank scenario, and you get the idea!
