A goldfish farm is a useful project which can generate an unlimited supply of goldfish at a decent pace. To make one, you will need three goldfish statues, some red wire, a 1 second timer, and a bucket of lava.
Getting the statues is probably the hardest/most time consuming part. Just create new, small worlds and use a speed-run to get through the dungeon, ignoring everything except statues.
Once you have three of them, make a 8x5 hollow box out of any solid material (stone or dirt will do fine). The inside of the box will be left at 6x3. It makes things easier if you leave room to walk underneath of it as well. Place the three statues inside, and pour a bucket of lava in the center - it should spread out, leaving a thin layer of lava at the bottom of the box.
Now, just connect the statues to a one second timer, and stand below the box with an empty inventory.
This will generate 3 goldfish per second, each selling for 2 silver. That makes for 6 silver a second, or 3 gold 60 silver a minute, or 2 platnium 16 gold an hour.
It should also be noted that during a Bloodmoon, the statues will spawn Corrupt Goldfish, which will drop 5-7 gold coins each, approximately tripling the speed of the generator for the duration of the bloodmoon.