Depends. The rule of thumb I go buy, in some cases give or take, is 55G for one, ONE Common Goldfish, and 30 for ONE Fancy Goldfish, like a Moor or an Oranda.
If you have multiple Goldies, you can chop of a few gallons. Like for my pond, each fancy will get 25 gallons, bit there will be a few empty gallons so I guess thats for them anyways.
And 75 for a Pleco.