The fish you catch vary by location in a variety of ways. Some fish can only be caught in the ocean/sea (east of the coastline) , others in lakes, others in rivers or a mix of the above. They also sometimes vary between deep and shallow water.
Depending on the spot you fish in, each type of fish will vary in alchemical composition, potentially being really pure. However, unless you have the necessary F&W you might catch say, 5% fish in a spot where normally they'd be 10%. The "fields" by which fish types vary in purity are independent for each type as well, so a spot where angel-winged seabass is merc, might also have another type of fish of salt purity.
You will also need different baits/lures to catch the different fish. As for the rods, I think except for the fishmaster 1600 they can all catch the same types of fish, the only difference is how fast they decay (which is also dependant on F&W i think).
The wiki's fishing page is a bit outdated, but check it out nonetheless to get an idea of how it used to work and might still work.
http://salemwiki.info/index.php/FishingExactly 6.022 x 10^23 worth of Lach molecules.
