Why aren't my bullfrogs growing? 1 year and still the same size

[clintosaurus rex]

It has been a year and a half since my bullfrogs ?metamorphed? and grew one inch in that time(1.5 to 2.5 inches). I know for a fact they are bullfrogs, and have heard some would be 3 inches at 4 months. What am I doing wrong?

Tank setup: 20 gal (24x12x16 high)
1 reg. cool white (bluish) flourescent light (no UVB)
Water heater (24 degrees Celsius)
8 inches of water
Exoterra 250 repti filter
T-rex log landing
3 fake plants

Inhabitants: 2 bullfrogs
10 white cloud minnows
Lots of pond snails

Feeding: Crickets and mealworms 2-4 times a week
calcium with D3 about every other feeding roughly
vitamin supplement about once a week, maybe less

Please tell me why they have hardly grown. I don't care if they even grow another inch, at least then they would be in the average size range.

[Locascio]

you need to try and power feed them i would offier food every day and try getting earth worms horn worms and anything els thats there size i have 2 that are huge thay eat chickens and small rats i hade the one sence she was 3.5 inch and shes almost 12inch long try mixing up there food thay might not be getting enuff

[clintosaurus rex]

Sounds like a good idea. How often should I supplement? Every feeding, every other feeding, calcium or vitamin? And up here in Saskatchewan, it is hard to come by diversity. Crickets, mealworms, superworms and waxworms are the main insects here, and the superworms only come in one size, and thats too damn big lol. They are as big as the frogs! I will go have a look though as sometimes the odd suff like butterworms, hornworms and pheonix worms come in. Maybe I should start bulk ordering instead. Btw how big of crickets should I offer? Right now I offer 3/8 to 1/2 inch crickets, and dump 20 in all in the tank.