What can I do to help my goldfish?

I have had a goldfish for about a month and a bit. It was really healthy and swimming around for that time. I installed a filter and had an aquatic plant. The plant died because the goldfish would uproot it and then it wouldn’t get enough light. I removed the plant about a week ago. For the last few days, my fish has been sitting at the bottom corner, completely still besides the flapping of its gills every time it breathes. It wont eat the food I give it. Im worried that it may die soon, is there any way to diagnos its sudden stillness and loss of apetite? Is there anything that I can do to treat it?