Our oscar Ernie was very sick a month ago - struggling with breathing-not eating etc
I fixed the problem by adding two air stones-apparently because of the increased heat here in Queensland which was a quite sudden this year- he was not getting enough oxygen. What has now happened is two things:
1.He is so hungry that is wants to eat all the time
2. He has changed colour -he was purple and gold -my husband says copper and brown and now he is completely black

I am feeding him Cichlid staple

Can anyone please advise me what to do ?

Welcome to the site

Oscars are always hungry. The fact that your Oscar is eating is a good sign. The first thing you must do is check your water conditions. You need to check for ammonia, nitrite, nitrates and pH. Fish can change colour, especially go darker if they want to be inconspicuous because maybe they are not feeling too good. However, like I said, your fish is eating so I don't imagine it's feeling ill.

What if the temperature of your water? Have you got any other fish? if so, is there any bullying going on? What size tank have you got? Have you got another aquarium that you can put the Oscar in just to see if it changes back to its normal colour again?

the fact that you had just completed medicating the tank, it has some way or
another effect on coloring... plus oscar really do change color based on:
excitement; stress, and water conditions....but as long as it is eating
well...no need to worry.