Over several days to a week, soften the pellets gradually less and less each day until fish are accepting hard pellets. Then if they will not eat the hard pellets you did not do the pellet soft to hard process long enough number of days. Another option is to soften just one size brand of pellet. Then when they are eating a hard pellet you can feed both brands/sizes of hard pellet. Note - if you are using a Tractor Supply nonAquamax brand pellet of 36% protein they do not soften properly - IMO. My experience is the cheaper brand of 36% protein pellets do not have enough flavor enhancers to encourage 'fussy' fish to eat hard pellets. They are rejecting cheap pellets due to flavor and texture.