Welcome, and ditto to all the questions and comments that have already been made. Most of the time when there is a problem, it has to do with water quality, one way or another. And problems with water quality can be caused by lots of different factors: over crowding, over feeding, growth of small fish into larger fish, inadequate filtration...
That's why the first thing most of us will do is ask all these questions regarding the size of your pond, #of fish, type of filtration, etc. But the most important thing we'll ask about is your water test parameters (already covered). Do you have a PetCo or PetSmart close to you? They carry the test kits, and as already mentioned, they will usually run your test for you if you take them a sample of your water.
Don't give up!