Harold may just not be a betta who will tolerate any tankmates. It is unfortunate but some bettas are like that. You see, they are raised with no other fish or tankmates at all in with them and they do not realize that they must share their water.
The only thing I have heard that even may help is to remove Harold from the tank and put him in a cup of tank water for a few minutes while you rearrange the decorations and all in the tank and make it look totally different. Sheldon can stay in the tank, in fact he HAS to stay. Then before Harold gets totally cold put him in a baggie and float him and reacclimate him to the tank temperature and let him back into the tank. Hopefully, he will think he has gone to a new home and must establish a new territory and get along with the new neighbors.
This is my only suggestion. I do know that he cannot hurt Sheldon as Sheldon will just pull into his shell if he is tired of Harold being a bully and ignore him. Snails will not put up with it. They just ignore it. Harold will take a flying leap at Sheldon one of these times and find that the shell is HARD and that should be the end of that! But he will have to learn the hard way if the rearrangement of his tank does not work. The only other way is to get a small tank and seperate them and you already have said that this does not work for you at present. But do not discount Sheldon's ability to care for himself. He is able to handle Harold okay.
Rose
