Well, for straters, to catch any fish you'll need to be looking in a river. I hear that Bass like rivers with water, so thats probably your best bet
You Find Feebas On One Of the Random Six tiles changed whenever the trendy phrase changes on route 119.
You cannot find a Feebas on the game but you can find a Feebas on the Pokewalker Route Quiet Cave
Mt.Cornet (There are 4 fishing spots.)
you can find a feebas in route 118 and 119 but its hard to find
find it in seafoam islands contest if u win u get feebas
you will find feebas in the water of grass maps in pokemon vortex
U can find feebas in Route 119.
to find a feebas in Pokemon diamond is very simple ( but you have to beat the league and get a national pokedex). You have to have a super rod (found at the survival area) go to the the fourth flour in mt.coronet or when you get to the first snow place you see, go to the first cave you see and there should be a place with a waterfall and water. use a super rod there and after a long look you might find a feebas.
Feebas are tricky to catch you can only find them in certain floors in Mt. cornet.
you have to go to a place in mt coronet where there is a lake a it is foggy. If you have defog use it. You have to use your old rod to get feebas in the lake. It worked for me after a few tries. Hopes this helps!
in the water
you cant find it you have to evolve feebas