If you have to leave fish unfed for a few days to a week or more, do the following things before you go.
Feed the fish 3-5 times a day for a couple of weeks before you go away. This will let the fish gain some weight and they can live off the fat reserves. Use a variety of dry, frozen and live foods.
Do big water changes and gravel clean the substrate every day or two when feeding more. This will keep the water cleaner and reduce the chance of diseases or poor water quality.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter a couple of days before you go so it is clean and there is less chance of it blocking up while you are away.
Feed the fish just before you walk out the door and they will be fine for a week without any more care.
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You can also add a heap of live plants a couple of weeks before you go, assuming you don't already have lots of plants in the tank.
Increase the lighting period to 16 hours a day for 2 weeks before you go and while you're away. It will encourage plants and algae to grow and the fish can pick at that. Reduce the lighting period when you get back.
Have the lights on a timer.