died fish

Question
What are the signs of a fish that is short on oxygen?
And do they react in anyway that tell what is going on with the fish.  Signs i should be looking for? 10 gallon tank with air stone. good filteration and still dead fish?  It happen very fast 1 died fish after another.  I done 20 percent water change every week. I had platys and it wasn't over crowed.  And i didn't overfeed them so i really don't know what i did wrong.  I'm very new at this and done alot of reading online about the cycling process and i was doing fine.  Fish lived fine for 2 months and all at once the dread of died fish..  I'm using bottle water distilled and thats because i live in an old house and the pipes are not at all in good shape..  Nothing is added to my tank It is laking something  Please help  Thanks

Answer
Hey :)

When fish are short of oxygen they rise to the top and of water and get air.

Sometimes fish die without youreven noing. One of the reasons that your fish keep dieing is because your pH could be out of balance. Also you could be under feeding them.

Hope this helps

Chloe xx