geeez it has to be oscars, i purchase a 1 1/2 inch oscar and a 1 1/2 inch frontosa (for the grow out tank) both the same size in a month this oscar has more than doubled the frontosa its leaving him in the dust... wow, picked up two tiny flowerhorns and i was concerned the oscar would try to eat them so i took a chance and i was actually able to put the oscar in the large tank with very little problems, he is adjusting well and i swear he just grew as i typed this... lol
he would definitely outgrow a front because 1) they are naturally slow growers and 2) you are housing the frontosa in the wrong water parameters. oscars and frontosas do not originate in the same type of water and require different water parameters in order to promote natural healthy growth. 3) oscars from 1" to 7" - 8" are one of if not the fastest growers