Especially in a tank as tiny as 45 gals, nitrate and hormones build up quickly and can slow growth.
The only way to get rid of those growth inhibitors is with frequent, large water changes.
Breeders that want to get fast growth, and to get cichlids to market size quickly, will often do daily water changes of 75% or more.
In nature, GTs come from the rivers west of the Andes where they are in 100% water changes every hour, and nitrates are undetectable.