An inch a month is normal for me untill the 6-8" mark then flips back and forth form bulk to length. As with most species water quality and diet will play a roll. Also the stress level can play a roll. If a fish is harrased and chased all day and can not eat as much as he would like that could affect the growth....that is why there are runts. Many facters but some things to think about. I have never had aslow growing Dovii so your set up is probley the issue.