Hi & welcome!!! The black 'bug-eyed' fish is a black moor. You didnt describe the other fish. When you say its a small tank, exactly how big is it? Fancy goldfish need a minimum of 20gal for the first fish, 10gal for each fish after. They would need atleast a 30gal tank with alot of filtration & oxygen for them to be happy & thriving. Goldfish are prone to swimbladder issues but poor water quality can make them worse.Have you checked your levels of ammonia, nitrites, & nitrates? What are your exact readings? For food-related swimbladder issues, feeding lots of cooked, plain greens (deshelled peas, broccoli heads, zucchini, spinach, etc) regularly along with a quality sinking goldfish food (such as Hikari) can be enough to alleviate symptoms. Non-food related swimbladder issues are usually due to an internal bacterial infection or parasite issue &can be addressed with a metronizadole medicated food (such as metromeds). Unfortunately, due to the physical build of goldfish, some swimbladder issues are of a genetic/poor breeding design & can not be cured with diet or meds. Please let me know tank sz & water parameters & we can go from there!