Now, simply double click th fish in your inventory and a window like this will pop up.

This is a random lottery with not equal chances on each slot, it works a bit like the roulette in the casino, the pointer can go back and forth, not only in one direction. I'll explain each reward slot now going from Top (12 o'clock):

12 o'clock: Random No idea xD
1;30 o'clock: Random Weapon (Normal, Rare, not sure about other types)
3 o'clock: Random Enchant Stone? (didn't get this yet so I'm not sure)
4;30 o'clock: Random Armor
6 o'clock: Random etc item (Monster claws, blood etc, just some shit)
7;30 o'clock: Random potion
9 o'clock: Random Random Slot? (not sure)
10;30 o'clock: Random Shield

Extra Fish Tips by golladan
The Rainbow Trout is a special fish that you have a very low chance of getting. It has no level because the only thing it can give are option stones or skill stones.

The item at 12 o'clock are option stones.

Each item belongs to a rank. Ranks can go up to 44. Maybe higher. When you use an item for fishing, that rank will be used as base.

If you catch the lowest level fish(the one that will show the rank in Red), you will get an item that is 1 rank lower than the one you put in. Basically, you'll get an item that is worst.
The next 4 fishes will give you an item of the same rank. You might even get the same item you put in, but with different(maybe better, maybe worse) magical stats.
The next 2 fishes will show the rank as Green, and will give you an item that is 1 rank higher than whatever you put in. You'll get a better item.
Then the next fish will show the rank as Yellow, and will give you an item that is 2 ranks higher than whatever you put in. You'll get rohan crone better item.
The very last fish, like I said before, is a special rank-less fish that will only give option or skill stones.

Also, most ranks can end up just giving you potions(and potions can't be used for fishing), so you might end up loosing your item you could say.

Lately, I've noticed alot of people selling fishes for you to try your luck.
*Warning* Before buying fishes from other people, check what Rank the fish says. Rank 0 Fishes will ALWAYS give the brown items that have absolutely no use other than selling to NPCs for very little money. Rank 1 Fishes will only give the lowest level weapons and armors(beginner weapons and leather armor). Elven equipment and stuff around that level doesn't start appearing until Rank 4 or 5.