Boredom is an emotion and you get on shaky ground when you start ascribing emotions to animals, especially ones so different from us. We are close enough to apes that we can often guess their emotional state, although we even can get that wrong with chimps. They see a smile as aggression, for example.
Fish have much simpler brains than we have and may not even have the hardware for emotions. That being said, it could be argued that it is cruel to keep a fish contrary to its social needs, if it has any. Many aquarium fish such as tetras, rasboras and Corydoras catfish are schooling fish. Their security comes from being part of a school. Not providing them with a school can make them behave abnormally. Other fish are solitary except at spawning time. Some will tolerate non-related species but don’t interact with them. They may be fine being alone their entire lives. Bettas are an example of this type of fish. If you put two Bettas together and they are not ready to spawn, one will likely kill the other in an aquarium. Cichlids, on the other hand can have complex social structures. Mbuna from Lake Malawi need to be crowded in an aquarium to defuse aggressive behavior. Keeping too few will result in bloodshed. Other cichlids form bonded pairs and apparently not any other fish will do. Some individuals just don’t get along. Some people keep large cichlids as single specimens in aquaria and they frequently interact with their keeper.
Personally, I don’t think fish get bored, just like I don’t think dogs can get bored. I believe that all of these animals live in the moment, without the concept of time. Watch a dog. When all their needs are satisfied, they just lay around. However when they receive the right stimulus they are up barking at the mailman or neighbor walking their dog. Or they are at the door ready for their walk. When the stimulus ends, so does the behavior.
The same could be said for a fish. It exists in a neutral state, awaiting stimuli. Take your Betta in its bowl, for example. When it is dark, he sleeps. When morning comes he awakens. If he is hungry he explores his surroundings picking on the biofilm that grows on every wet surface. When you add food, he eats. If he is healthy and the temperature is high enough he builds a bubble nest. If he sees a female Betta, he courts her. If she is receptive they spawn, if not he drives her away. If there is no stimulus, internal or external, he simply exists.