This thread did get derailed, but I feel I need to clarify a few things instead of people speaking for me.
I love my fish. And I came here to ask for help and opinions, and I appreciate ALL of the opinions given, but certain responses did feel a little rushed and/or harsh. It's my fish, and obviously I'm going to be the only one who cares about it, but there is a general show of respect to the owner by showing empathy. I'm not mad at any of the responses given to me, if anything I'm disappointed by the lack respectable disagreement shown when a highly debatable topic comes up.
Gus should have been culled from birth. That is a fact. He's nothing but bad genetics, but I do still care for him.
I would love to bring him to a vet and have his tumor looked at, and while it's something I've looked into, it's not something I can realistically do right now. I'm a college student who can barely afford her books, I don't have the money to spend on him.
In the end, I think I will euthanize him. I've had blind fish before, and while he manages, he does not thrive or even succeed to the same degree my others have.
Even if his tumor is noncancerous, there's no telling if another problem would arise in the future. Overall, he's not healthy, and I don't think he's particularly content either, and I certainly don't feel like a good fish mama when I see him, and even though I'm doing the best I can, it's not the specialized care he deserves.
Because he is not yet suffering, as far as I can tell, I will wait a few more months until spring comes and I have a place to bury him.
Thank you everyone who replied, and a special thank you to those of you who gave obviously considered and thought out responses.