As mentioned, swim bladder problems are a common cause of floating fish, especially in fancy varieties with deep bodies, but a swim bladder rarely malfunctions for no reason. High nitrate levels are the main cause of these problems. This suggests to me that your tank may have cycled but you're not doing enough water changes. Once you have enough beneficial bacteria in your filter, the ammonia your fish excrete will be converted to nitrite, then to nitrate and it's this nitrate that we aim to remove in weekly water changes.
25% weekly is the general rule of thumb but to remove the high levels you already have in your tank, I'd do a 50% change this week and 25% every week thereafter. Fasting for a day and feeding shelled peas will definitely help in the short term too.