I wonder if increasing the temperature of the water would help or hurt. Generally, increasing the water temperature (and increasing oxygenation, additional current/filtration and or water changes) increases the metabolism of fish, so increasing the water temperature along with food intake would theoretically increase the growth rate.
However, you have blue phantoms, and one school of thought is that these fish are the same as the green phantoms, but from different populations based on water temperature (blues being from cooler water). Could increasing the water temperature also affect the color of these phantoms?