I have an issue of hardening of fish food in open jar.
Now you may ask, why you kept the jar open, its because , this jar is a automatic fish feeder.
See here 
So the compartment rotate with top side always open and when the compartment reaches down, food falls from it.
But this food inside compartment geting hardened. Anyway to prevent hardening of food?
Do you mean drying or caking?
If you meant drying: You would have to change the recipe or use another fish feed or put less feed but more frequent feed into the automatic fish feeder. Regarding changing the recipe: You can add humectants but be aware that some are not suitable for the use in tanks (because they are water soluble and would change the water properties) or are directly harmful for the fishs consumption. For human consumption
are used as food additives.
Concerning the non-ionic polyols Wikipedia says (emphasis mine)
Some of these humectants are seen in non-ionic polyols like sucrose, glycerin/glycerol and its triester (triacetin). These humectant food additives are used for the purpose of controlling viscosity and texture. Humectants also add bulk, retain moisture, reduce water activity, and perform the important function of improving softness. A main advantage of humectant food additives is that, since they are non-ionic, they are not expected to influence any variation of the pH Aqueous systems.
If you meant caking: You could try to use some anti caking agents. Same thing like before: Please do research whether the agents are (in which concentrations) harmful for the fish.
