ok, my advice:
tank maintenance
- weekly waterchanges of about 30% of the total volume.
- never change with ice cold water directly from the tap, always about the same temperature as the tank. if possible, put water in a clean bucket (one that has never seen soap or detergents) for a night to let chlorine and such evaporate.
- rince out the filter every 2 weeks under the tap. change the sponge inside every few months.
-mount the filter so that it gives some current movement on the surface. more movement means better oxygenation and better water quality.
the fish
- I wouldn't put in more than 2 goldfish. they're big polluters.
- feed them every 2-3 days and only a little. no more than can be eaten in a minutes time. it seems little, but goldfish are able to eat all the time and the more you give, the more they pollute.
This is not a shoe-in for success, but it's common sence advice that I give any aquarium beginner. I'd say good luck!