When you fish, you are able to choose the level you'd like to fish at. This level is what determines what fish you catch and also whether or not you get experience.
The minimum level you get experience at is 1+(fishing rank), so if you are rank 5 fishing you will get experience for any level greater than or equal to 6.
If you want to catch T2 fish you will have to fish at level 5 or higher, T3 fish at level 10+, T4 fish at level 15+, etc. Even if you fish at level 5/10/15+ you will still catch the lower tier fish like sardines unless you are at a well. Fishing at a well allows you to catch only the 3 highest tiers of fish for the fishing level you're fishing at, so at level 15 you will no longer catch sardines, level 20 you will no longer catch sardines and herring, and so on.
As far as which rods you can use, I believe they follow the same level requirement as the other items do. So T1-3 require rank 0+ fishing, T4 requires rank 5+, T5 requires rank 10+, etc. I also believe that it's more efficient to use a rod at their respective monster level as seen here:
http://ladderslasher.d2jsp.org/index.php?g=14#Wood For example, you should use pine for fishing levels 5-9 and for 10-14 you should use oaks, etc.
level 20 you will no longer catch sardines and herring...i thought it was every 15 ranks for the min required fish to not spawn anymore