Im always astonished when I hear about frogs that are an inch and a half at 4 months. I have a female that was quarter sized when I got her in the very end of June. She is well over four inches.
What kind of feeder fish have you been using? I have found that despite the fact that these frogs eat primarily vertebrate prey in the wild, there is very little in the way of vertebrate prey offered in the pet trade that makes for a safe or healthy vertebrate diet. Feeder fish would be a great food item if not for the risk of disease and exposure to antibiotics in feeder fish. The poor conditions these fish are kept in make them an unsafe choice. Pacman food makes a great alternative, and its base ingredient is fish meal.
Considering their rapid growth rates and the fact they they do indeed eat vertebrate prey, Pacman frogs need lots of calcium to grow and maintain health. Insects, particularly crickets are very low in calcium and have a poor calcium to posphorus ratio. This can be made up for by supplementing the crickets with a calcium and D3 supplement at every feeding for a juvenile. For adults I use Calcium with every feeding, and calcium with D3 at every other feeding, as D3 overdose is possible.
Reptivite is a reptile and amphibian vitamin that includes D3, use it as directed. For a young frog I would use calcium at every feeding. The cheapest and most readily available source of calcium carbonate would be to get plain, unflavored tums and crush them up and dust the food with them.
For a younger frog, or one that is still rapidly growing, its generally better to keep them at the more intensive temperature of 80 to 85 degrees. For a more matured frog, between 77 and 82 is perfectly fine.
When seeking advice on Horned Frog care, I generally look toward the advice of both Phillipe De Vosjoli and Robert Mailloux, who played an important role in establishing Horned Frogs in captivity and have extensive experience with these animals.