Hi hopefulgirl,

!
We have a saying around Kokos: take care of your water and it will take care of your fish. This means that we are really all water chemists, not fish keepers!
Your fish is probably floaty due to poor water quality. 10 gallons is not nearly enough for 2 fantails and a pleco. Goldfish of any type and
any size need at least 10 gallons of water each. They also need strong filtration-- 100gallons per hour per 10 gallons. The ideal setup for you would be a 30-40gallon tank with a filter that processes 300-500 gallons per hour.
Have you read up about the nitrogen cycle?
http://www.kokosgoldfish.com/cycle.htmlIt is really vital to know what your water parameters are at all times. How long has your tank been running? It's possible that your tank isn't even cycled, and toxic ammonia and nitrite can climb to deadly levels. You should get a drop test kit ASAP and let us know what your parameters are.
I would do a BIG water change ASAP. If ammonia, nitrites, and/or nitrates are high, sensitive fish can go belly-up. You should be doing large water changes every 3 days to keep the levels low.
You said you don't add chemicals to your water. I hope this doesn't include water conditioner. Tap water contains toxins and heavy metals such as chlorine and chloramine which are DEADLY to fish. Adding a water dechlorinator that neutralizes these, making the water safe for fish. (You can also evaporate them by leaving water to sit for 24 hours.)
You should also think about getting a larger tank for them. If you can't afford one, a large Rubbermaid tub works great as a temporary tank. Getting a better filter that filters more water per hour will also help.
Good luck, and let us know how it's going!