Eating The Symbolic Foods
The Maharshal did not eat fish on Rosh Hashana. Although he was fond of fish, on Rosh Hashana he abstained from his favorite delicacy. Why was this? After all, the Gemara says that fish is one of the symbolic foods we should eat on Rosh Hashana.
This statement holds true, however, only if we eat the symbolic foods with the purest of intentions, solely for the sake of the mitzva. But if we eat these delicacies because we enjoy them, we are merely satisfying our physical desires, and the symbolic value of the food is lost.
Since the Maharshal enjoyed eating fish, he was afraid that he might not eat them for the sake of the mitzva. Rather than indulge in mundane pleasure, he did not touch the fish.