Increase the growth rate..what fish?
You dont have to do that many water changes to grow fast healthy fish.My fish freak out everytime I do a waterchange(except the barbs and shrimp), but they are so happy afterward.
Keeping the nitrAtes low is one thing.Weekly or twice a week at a bigger percent is fine.
I do a 25-30% once a week on my 29g planted, 40 to 45% on my 55 1/2 planted( a lot of fish), and a 50% bi-weekly on my 10 g.
Not over populating, keeping the fish load low to moderate will help.
More feedings(in smaller amounts) will help. If you feed too much too often, your fish will get big..but real fat..and unhealthy. My mother-in-law has this problem with the 55 gallon that I let her have now. Feeds once a day...but way too much. her fish are twice as big as mine..and they are so fat they look silly.
Keep your fish in a tank that is bigger than they need, will help them grow..and help them reach max size.
All I can think of at this time.