Unsuitable foods. Freeze dried food will cause constipation, bloat, and is nutritionally vapid. Wet/Frozen foods are much better. Frog pellets should be okay though you did not specify the brand and it would help if you could do that. Guppies are a real gamble, as are most feeder fish, especially to a frog which is more than likely already stressed they are a fantastic way to introduce parasites, bacteria, disease, ect..
5 1/2 gallons is unsuitable for Xenopus Laevis. These frogs require 10 gallons each. The issue here is not stunted growth...
The REAL issue is the fact X. Laevis is a an animal which requires very little basic care so long a FEW, SIMPLE NEEDS ARE MET.
Please do read on proper care for your frogs [link].
My best advise for you is to purchase a 20 gallon long aquarium which is perfectly suitable for two adult frogs. Get a right fitting lid on top so that the frogs cannot escape, basic lighting with a 12/12 hour day/night cycle, and place in the tank a filter capable of handling the frogs heavy bio load (ie: a filter rated up to 40gallons or 80GPH).
Feel free to leave the bottom of the tank bare for easy clean up, add some live floating plants so that the frogs may bask in them. For this water sprite/indian fern (or any other suitable floating plant) works well, it will soak up some extra nitrates from your frogs bio load and provide them a place to rest.
For feeding any suitable frog pellet will do, I use reptomin personally but there's a lot of options here. Earthworms (nightcrawlers) are a particular favorite for these frogs as well. Make sure excess food is REMOVED after feeding and only feed these frogs every other day or even every 2 days. These are subtropical frogs and their metabolism is much lower than most frogs, also note optimal water temperature would be 68F to 72F so a heater is not necessary so long as your houses temperature is within that range.
I ask that you please take my advise and read carefully, these frogs are most likely stressed from extremely unsuitable husbandry and the quicker you can rectify this, the better.