If Clue 4 is:
Green House is to the left of, and next to the white house. (This means GW are neighbors)
Then:
German has Fish.
Solution is:
(1, Nor, Yel, Wat, DH, Cat)
(2, Dan, Blu, tea, BL, Hor)
(3, Bri, Red, Mil, PM, Bir)
(4, Ger, Gre, Cof, PR, Fis)
(5, Swe, Whi, Bee, BM, Dog)
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But in this riddle, because normalman did not mention "and next to" in clue 4, I should suppose G and W may not be neighbors.In this case, there are 6 more solutions which can satisfy all 15 clues. Any of Norweigian, Danish and German has chance to have fish.
Complete Solutions are:
Solution 1:
(1, Nor, Gre, Cof, PM, Bir)
(2, Gre, Blu, Wat, PR, Fis)*
(3, Bri, Red, Mil, BL, Hor)
(4, Dan, Yel, tea, DH, Cat)
(5, Swe, Whi, Bee, BM, Dog)
Solution 2:
(1, Nor, Gre, Cof, PM, Bir)
(2, Gre, Blu, Wat, PR, Cat)
(3, Bri, Red, Mil, BL, Hor)
(4, Dan, Yel, tea, DH, Fis)*
(5, Swe, Whi, Bee, BM, Dog)
Solution 3:
(1, Nor, Gre, Cof, BL, Fis)*
(2, Gre, Blu, Wat, PR, Cat)
(3, Swe, Yel, Mil, DH, Dog)
(4, Bri, Red, Bee, BM, Hor)
(5, Dan, Whi, Tea, PM, Bir)
Solution 4:
(1, Nor, Gre, Cof, PM, Bir)
(2, Gre, Blu, Wat, PR, Cat)
(3, Swe, Whi, Mil, BL, Dog)
(4, Bri, Red, Bee, BM, Hor)
(5, Dan, Yel, Tea, DH, Fis)*
Solution 5:
(1, Nor, Gre, Cof, PM, Bir)
(2, Gre, Blu, Wat, PR, Cat)
(3, Swe, Whi, Mil, BL, Dog)
(4, Dan, Yel, tea, DH, Fis)*
(5, Bri, Red, Bee, BM, Hor)
Solution 6:
(1, Nor, Gre, Cof, PM, Bir)
(2, Gre, Blu, Wat, PR, Fis)*
(3, Swe, Whi, Mil, BL, Dog)
(4, Dan, Yel, tea, DH, Cat)
(5, Bri, Red, Bee, BM, Hor)
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BTW, after I solved this riddle, I ran an internet search, and found out the real Einstein Riddle DOES mention "and next to" in clue 4.

So, normalman, please make sure you get the RIGHT riddle when you post any in the future.