What does it mean if your fish is laying on its side but still breathing?

The fish is dying and will die if you don't change its water ASAP. There are some basic rules for keeping all species of fish. They are. :- 1 inch of fish needs at least 1 gallon of water. (Goldfish are 10 inch fish) :- Every tank needs a permanently running cycled filter. (This turns Ammonia into Nitrate biologically). :- Every tank needs at least 50% of its water replaced every week. (This keeps the Nitrate levels under control). If you keep to all the above rules your fish should do OK. If you don't I can guarantee that your fish will constantly be getting sick and they will have a shortened lifespan.