Let's solve this...
I'll use the clues out of order. Here we go:
9. the Norwegian lives in the first house.
14. the Norwegian lives next to the blue house
this means the blue house must occupy position #2, while the Norwegian is in position #1. (1 being far left, 5 being far right)
1. the Brit lives in a red house
4. the green house is on the left of the white house.
since the blue house is in position #2, for the green house to be to the left of the white house, the green house and the white house must occupy either positions #3 & 4, or positions #4 & 5. And since the Brit lives in the red house (and not the Norwegian), then the red house must occupy either position #3 (if the green & white houses are #s 4 & 5), or position #5 (if the green & white houses are #s 3 & 4). And by elimination, the yellow house must be in position #1, meaning that the Norwegian lives in a yellow house.
5. the green house owner drinks coffee.
8. the man living in the house right in the center drinks milk
The sum of these 2 clues means that the green house cannot be the center (#3) house. So by elimination, the green & white houses must be in position #4 & 5, meaning the red house is in the middle (#3) position. This also means that the Brit lives in the red house and drinks milk.
7. the owner of the yellow house smokes dunhill.
11. the man who keeps horses lives next to the man who smokes dunhill
That is the Norwegian who lives in the Yellow House (position #1) who smokes dunhill. And that means the man living in the blue house in position #2 is the one that keeps horses.
2. the Swede keeps a dog
We know that the Norwegian is in the Yellow House (#1), the owner of the Blue House (#2) keeps horses, and the Brit lives in the Red House (#3). So this means the Swede with his dog must live in either the Green House (#4) or the White House (#5). We'll come back to this later....
3. the Dane drinks tea
We know that the Norwegian is in the Yellow House (#1), the Brit lives in the Red House (#3), and the owner of the Green House (#4) drinks coffee. So the Dane who drinks tea must live in either the Blue House (#2) or the White House (#5). We'll come back to this later.....
12. the owner who smokes blue master drinks beer
We know that the Norwegian in the Yellow House (#1) smokes Dunhill, the Brit in the Red House (#3) drinks milk, and the owner of the Green House (#4) drinks coffee. So this owner who smokes blue master and drinks beer must live in either the Blue House (#2), or the White House (#5).
Now look at the last 2 parts in bold. This means that the Dane who drinks tea, and the owner who smokes blue master and drinks beer must occupy the Blue House (#2) and the White House (#5).
15. the man who smokes blend has a neighbor who drinks water.
Since we now know that the owners of the Blue House (#2) and the White House (#5) drink tea & beer, the Brit living in the Red House (#3) drinks milk, and the owner of the Green House (#4) drinks coffee, this means that the Norwegian living in the Yellow House (#1) must be the one who drinks water, and thus the owner of the Blue House (#2), his only neighbor, must be the one who smokes blend.
Now above we bolded the following: So this owner who smokes blue master and drinks beer must live in either the Blue House (#2), or the White House (#5).
Since we now know that the owner of the Blue House (#2) smokes blend, the owner who smokes blue master and drinks beer must be the owner of the White House (#5).
Above we bolded the following: So the Dane who drinks tea must live in either the Blue House (#2) or the White House (#5).
Since we now know that the owner of the White House (#5) drinks beer, then the Dane who drinks tea must live in the Blue House (#2).
13. the German smokes prince.
Since we already know that the Norwegian lives in the Yellow House (#1), the Dane lives in the Blue House (#2), the Brit lives in the Red House (#3), and the owner of the White House (#5) smokes Blue Master, this means that the German who smokes prince must live in the Green House (#4).
Above we bolded: So this means the Swede with his dog must live in either the Green House (#4) or the White House (#5).
Now by elimination, the Swede with his dog must live in the White House (#5).
6. the person who smokes pall mall keeps birds.
Since we already know that the Norwegian in the Yellow House (#1) smokes dunhill, the Dane in the Blue House (#2) smokes blend, the German in the Green House (#4) smokes prince, and the Swede in the White House (#5) smokes blue master, then by elimination the Brit in the Red House (#3) must be the one who smokes pall mall and keeps birds.
And for the last clue:
10. the man who smokes blend lives next to the one who keeps cats
We know that the Dane in the Blue House (#2) smokes blend. Since his neighbor on the right, the Brit in the Red House (#3) has birds, then his other neighbor on the left, the Norwegian in the Yellow House (#1), must be the one who keeps cats.
So for a summary of everything so far, we have:
Position #1:
Norwegian
Yellow House
Water
Dunhill
Cats
Position #2:
Dane
Blue House
Tea
Blend
Horses
Position #3:
Brit
Red House
Milk
Pall Mall
Birds
Position #4:
German
Green House
Coffee
Prince
(pet unknown)
Position #5:
Swede
White House
Beer
Blue Master
Dog
So to answer the question: Who keeps the fish?
Obviously the answer is the German. 