The Fountain is to the west of your crop field on your farm. The area has a fish pond where you can hold the fish you caught. To catch fish you need to buy the Fishing Pole from Bill, which is available at the beginning of the game. When you buy the Pole from Bill, he mentions maybe you can catch the Fountain King. What could he mean?
You can fish in the Fountain area, or you can fish at the river that runs through your crop field. To use the Fishing Pole, first have it equipped as your active item and then walk up next to the water. Press the A BUTTON and if you are close enough to the water you'll dip your Fishing pole in and sit down. To pull a fish up you need to wait until you see the end of your pole twitch and drop down into the water. At that point press the A BUTTON again and if you're lucky you'll catch a fish. You can also catch boots and cans, but they just disappear back into the water.
Once you catch a fish you can either put it in your fish pond for later or put it in the shipping bin for money. There are three different sizes of fish; small, medium, and large. Small fish sell for 40G, medium for 80G, and large fish sell for 100G each. There are 18 different fish you can catch. Each time you catch one for the first time, the fish's data is added to the Fish Picture Book that is located in Mary's Library.
The sign next to the fish pond lets you know how many fish you currently have in stock in the pond. To see what types of fish you have, or to take a fish out of your pond, you need to make sure you have nothing selected as your active item (in "empty hands" mode) and then press the A BUTTON while standing next to the fish pond. A listing of the types of fish you have and how many of each will appear. You can scroll through the list by using the UP and DOWN directional pad on your Gameboy. To take a fish out of your pond, pick a fish in your list and press the A BUTTON. You'll receive a confirmation message asking if you want to take that type of fish out of your pond. Select YES and you can take the fish and put it in your shipping bin if you like.
The fish pond can hold 99 of each different fish. If you try to toss a fish into your pond and you already have 99 of that particular fish it won't count for the total amount of fish. So if you have 500 fish and 99 of those are Bluegills, throwing in another Bluegill won't make your total 501. Also some fish are rarer then others. Fish are not seasonal either, so you can't catch more Carp in the Fall than you can during the Spring.
You can trade fish by linking with another game named Legend of the River King 2, a RRG/Fishing simulation game. That game is scheduled to be released in its English translation around mid January of 2001. Perhaps Bill's "Fountain King" story will be discovered at that time.
Below is a listing of the different fish you can catch, how much they sell for, and how rare they are on a scale of 1 (common) to 4 (really rare). The fish are listed the same order as they are listed in the fish pond inventory screen.