"So long and thanks for all the fish."
Translation:Hoşça kal ve bütün balıklar için teşekkürler.
I'm not certain, but I believe that would mean "thanks for the entire fish" rather than "thanks for all the fish" (the second one indicating that you received more than one fish).
It's a bit ambiguous in English, since the word "fish" can be either singular or plural: "That's a big fish!" / "He ate three fish."
When a word + için is used, it usually comes before the rest of the phrase that it is in - yardım için teşekkürler rather than teşekkürler yardım için. It may be more of a convention than a rule, but in Turkish modifiers that make something specific usually come first, so because it is specifying what you are thanking for, it comes before thanks.
"Teşekkürler" can stand on its own. (I don't know why, to be honest, but it's just like English: Thanks!). However, "teşekkür" needs an auxilary/helping verb ("etmek").
- Teşekkürler = Thanks
- Teşekkür ederim = I thank you ("sana/size" is implied)
You will find "etmek" popping up all over the place to help out. :-)
"So long" has two meanings in English. It depends on the context. It can be literally "I can't believe this is taking so long" or it can mean "goodbye." When meaning "goodbye," it is quite informal. As Selcen wrote in the comments above, So Long, and Thanks for All the Fish is the fourth book of the Hitchhiker's Guide to the Galaxy trilogy written by Douglas Adams, so this sentence is kind of an in-joke. There is also a famous song by Woody Guthrie entitled "So long, it's been good to know yuh" (you). https://youtu.be/zqiblXFlZuk
I notice two problems with your sentence.
First: "So long" is an informal English term that can either mean "goodbye" or "very long". Here are two sentences showing the difference:
"That cat's tail is so long!" (length)
"So long, see you tomorrow!" (goodbye)
In this question, the speaker is basically saying, "Goodbye, and thanks for all the fish".
Next: you've got the second part in the wrong order.
Bütün balıklar için teşekkürler.
All the fish for thanks.
All (bütün/tum) the fish (balıklar) for (için) teşekkürler (thanks).
You also forgot the "r" in teşekkürler, but Duolingo is often forgiving with spelling.