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I have noticed for the passed few days a lump on my bettas left side. It doesn't appear to be hurting him or anything. The only weird thing I have seen him doing is laying on the floor more.
If it's a tumor then no. Outside of performing a surgery. Keep his water pristine. 50% water changes weekly. If it's a parasitic cyst you could try erythromycin. Even then it's hard to cure. Sorry my friend.