"I do not like fish."
Yes! And you can do the same thing with 好き, make it 大好き and it's like "I love this!" Japanese is pretty logical...sort of like how 大学＝"big school" = "university".
I found that my ability to learn and remember Japanese was hugely improved when I started understanding the internal logic of it. It can even lead me to understand the meanings of words I've never heard before, if I know one or more of the components it's made of.
Looking up Kanji can also help a lot with this...which is one reason I think DuoLingo is causing us to miss out by not teaching some of the Kanji, since the Kanji often contain a lot of meaning.
I guess the way Duolingo is structured it'd be pretty hard to teach radicals and kanji well enough in here. I've taken a 3-year break from my Japanese studies but I used to heavily rely on Wanikani to learn kanji (I haven't seen anything better for learning kanji specifically so far). It's a bit troublesome to use different media to learn but it'd really help with appreciating the logic behind Japanese as cazort says ^_^
I also use (and love) wanikani to learn Kanji. It is like opening a whole new world once you start recognizing and reading Kanji. I was intimidated at first and honestly the first few days of Wanikani are painfully slow but then it gets going about day 4 or so. Well worth it.
Duolingo seems to stress a lot of negative statements that I'd probably refrain from in Western culture, and strike me as even less likely that I'd use in Japan.
It's not the nicest thing to say that you don't like something, and it's getting aggressive to say that you hate it, but it seems worse to me to tell your host that it tastes bad.
If I don't happen to like tea, I think a host can live with that, although it might come across as unlikely. But saying that it tastes bad means that I am not expressing my preference but making a blanket negative statement about what was given to me.
Duolingo keeps giving those sorts of statements over and over. I would hope to learn some more diplomatic ways of saying things. Am I wrong, and is it common to criticize in strong terms when there are other options?
You're using が because the subject isn't yet established to speaker or listener, you're saying you dislike fish in general, not a particular fish that has already been identified in the conversation. If you liked fish in general but then tasted one that was not good, you would use は.
さかながきらいです I do not like fish. さかなはきらいでさ I do not like the fish.
I'm not completely sure that is true. が is used here instead of は because the full sentence is actually 私は魚が嫌いです, just the 私は can be ommited. The は particle can also be used to emphasize negatives, meaning that we could say 魚は好きじゃないです and still get the same meaning using the は particle. This is not a negative phrase though, so we use が.