Some options are: (assuming you have Lu Shangs)
Nashmau Robber rig Istiridye (This is the shall shell equivalent which can be found in buburimu peninsula or Valkurm Dunes) by the docks until level 56ish. I personally did Shall Shells in buburimu until they capped, but that was only because I couldn't stand Nashmmau's dreariness, and I think a beach is nicer to stare at.
One thing you should know about fishing is you don't need to be anywhere near the level of fish you are trying to catch, which is why people reccommend Nashmau all the way from 0-90 just for simplicity. Shall shells are level 56 fish and with a lu shangs they are not all that hard to pull up and you can expect an average of 2 skill ups a day without rings. You could catch Black sole which are level 96 cap around level 30 fishing if you felt like it.
Anyways, from there you can use a shrimp lure in Nashmau and skill up off of Ahtapots(Grimmonite equivalent I think) and Mercanbaligi (Bastore bream). I personally stopped at lv 85ish and did Black sole to 96, Armored pisces to 107, Cave cherax to 110, assuming you will eventually want to cap.
Basically, there are many options out there as it doesn't really matter what you fish. Black sole NPC for 700 gil each and can be nice gil on the side while you skill up as I believe it turns out to be around 180k if you fish to cap. Leveling my mule atm and I will probably send here there around lv 30.
tl;dr
Lazy and simple way:
Nashmau 4 lyfe. Robber rig Istiridye to 56, shrimp lure to 90ish on Ahtapots/Mercs
Edit: This is a very good fishing resource and I use it almost daily:
http://www.great-blue.jp/?lng=en
Back to studying (`・ω・´)”