Why are there two distinct types of food? What is the right one for my needs? Mystical Creatures offers two kinds of food, Feracious and Arid Crystal. Both last the same time they can feed a dragon for four weeks.
If you are breeding your dragons, you will need the Feracious Crystals, as they are able to grow Ardor/Zeal your dragons need to mate. Ardor/Zeal will begin to rise if the mood is at 75% or higher. Once the Ardor/Zeal reaches 100% the dragoness will be ready to mate with any Drake that has at least 1 geno, if they are awake. Once they mate the egg drops immediately and will be ready to incubate.
There are cases you don't need to breed your dragons, and you want to keep them alive; in those situations, Arid food is the best choice to feed them.
You can change the food you are feeding your dragons anytime, BUT it will take seven days on Feracious Crystals for the effects of the Arid Crystals to wear off.