here are some fish I have caught, they helped towards my 5,300,000 fortune :) lol anyway here they are- 
at night when raining- coelacanth- 15,000 bell
Random time, near waterfall- arapaima- 10,000 bell
Random time, in stream- Gar- 6,000
Random time- Barred knifejaw - 5,000 bell
Random time ( but i get them all the time in my town) - Red Snapper- 3,000 bell
Stream- Angelfish- 3,000 bell
Koi- 2,000 bell
stream- Eel- 2,000
Goldfish- 1,000 bell
here is a list of the ones i caught today, just to show you how easy it is to make big money- 
arapaima
gar
red snapper
angel fish 
goldfish
throw in a fossil, thats all in an hour folks, and it came to 28,000 bell!!!
fish are a really good source of bells, as is shaking trees,
 100 bells in about 10-15 trees in your town. Also you can
 get furniture from the trees. Be careful though, bees can 
chase after you, when you get stung, don't worry, nothing 
bad will happen if you just leave it. If you manage to catch 
the bees they are worth quite a reasonable amount, the 
hardest bit is getting your net out in time! 
 happy bell making