need tanks as big as you can give them - a minimum of 20 gallons per fish, but even better with 30. That's with weekly 50% water changes.
I know that sounds insane, given that we all grew up with the fish in a bowl approach, but goldfish
can withstand horrible conditions. They shouldn't have to, but they can. They're cheap and expendable so they get the leftovers when it comes to care.
They should be long-lived, big fish, but that doesn't get to happen often. To me, they are outdoor pond fish for places that don't have deep freezes.
I teach in a high school that has a shallow pond (and Canadian winters). I used to keep seven goldfish over winter in a 75 gallon. It lasted three years, until a heron came into the pond one summer weekend and taught the kids about the circle of life...
From November until April, when I had those beasts in my classroom (with a huge and efficient homemade filter) I spent more time on maintaining those goldfish than I did on the Discus I had at home, by a large margin. Goldies are work, work, and more work. Your best algae eater is you scraping away...