Increase protein to 55-60% of their diet (if they are NOT mbuna).. You can do this by feeding them shrimp cut up and either cooked or raw. Another GREAT way to increase their size fast is feed them babies from other larger clutches. I do this with livingstonis; I breed them, raise them to 2" and then they get feed. I find this is the best way b/c I control the "live" feeding and I know they are not contaminated. Of course this is just for haps/peacocks as mbuna would get bloat very very quickly as their diet is 90% vegetable matter.
Raising the temp will also help, however, you will need to feed them 2x the amount to counter affect the new metabolism rate which then makes you clean the water 2x the amount.
My recommendations:
Change 50% water weekly and feed 2-3x daily of HIGH protein foods. Skip feeding for ONE day so they have time to clear their system out. This is how I get them grown pretty quickly.. Just got to make sure that they are a carnivore and not a vegetarian.