I have a male rose petal betta ,which I have had for about a year now, and I've recently noticed that it has trouble swimming. I keep it in a 5 gallon tank with 2 other glo fish (they are both fine). My betta usually just sits in one place for a long period of time. Sometimes it is floating sideways at the top of the tank without moving. I always think it's dead when this happens but, it starts moving again. Other times it's on the bottom of the tank laying sideways. I'm not sure if there's something wrong with its swim bladder or if its sick. Also I think its tail is different looking now than when I first got it and it has definitely lost its color. I'll try to upload some picture of it when I figure out how to.
The first issue I can tell you right now you have too many fish for your tank size. Betta do best when kept ALONE. If they are in a community tank they need to have plenty of space. 5 gallons is the minimum for a single betta. One. One betta.
Have you tested your water? Your overcrowded tank, unless you clean it regularly, is very likely to cause unstable water paremeters which lead to a stressed fish, which leads to illness. I recommend an immediate 50% water change and gravel cleaning . Make sure the water is treated with water conditioner and is close to the main tank's temperature as possible. A large difference in temperature can cause a shock and further harm your fish, not just your sick betta. Make sure to also pour the water in slowly, Betta are weak swimmers to begin with and a large water disturbance will also stress him greatly.
Once you have your tank clean you need to test your parameters. I don't recommend test strips as they are usually inaccurate. You can purchase an API freshwater test kit at most local fish stores like petsmart and petco for a reasonable price. You get more than a hundred tests out of it if I last remember, so you save money in the longrun vs. paying 5-10$ a box for a dozen test strips. I also recommend rehoming your glo-fish. If you have another suitable tank with room you can place them there. Even if you improve your water quality the amount of mainetance you will need to do to maintain this quality will be doubled vs. if you had just your betta.
Please list us your parameters. Nitrates, Nitrites, Ammonia, Ph, and Alkalinity.
Please also answer the following questions.
How much do you feed your fish? How often? What kind of food?
How often do you clean your tank? Do you vaccum the gravel?
How much water do you remove/replace when you do your water changes?
Do you have adequate water flow? Even though betta have labyrinth organs they need well oxygenated water just like other fish. If he's too weak to swim up for air this is especially important for his recovery.
At what temperature do you keep your tank? Betta are tropical fish and like water at about 78 degrees. Cold water can cause lethargy in any tropical fish. If you don't have a heater you need to purchase one.
Have you done any other changes to your tank recently? Added anything to the water than water conditioner, new decorations? A change in feeding quantity/schedule? A change in food?
I know this is a lot, but if you provide us with this information we can then go about advising you on how to improve your tank.