Just feed the fish more often (3 times a day) for a week or two before you go. This will allow the fish to gain some weight and build up fat reserves.
Do big daily water changes and gravel clean the substrate for a week or two before you go, especially when feeding more often. This will keep the tank clean.
Clean the filter a few days to a week before you go. This will make sure it doesn't block/ play up while you are away.
Then just feed the fish once before you go and leave it at that.
If you have some live plants in the tank it will help but a week without food is fine for fish, especially if they have been well cared for prior to that.
Make sure you have a cover on the tank to stop the fish jumping out.
Have the lights on a timer.