"The fish was slowly swimming in the river."
It's a different case of the verb.
およぎました = swam (So yes, it would be "the fish swam" if this was used like you said!)
およいでいました = was swimming. It's a combination of およいで, the で form of およぐ (to swim) and いました, the past tense of います, to exist.
The way I think of it is "In the past, the act of swimming existed" therefore, it becomes "was swimming".
GreggoGato has a good breakdown a few comments below as well.
(P.S. if this is wrong at all, please comment and explain why it is so I can come back and correct it so people can see the correct version. I'm still learning, too)
So for other examples it has been に in reference to fish swimming, location - ni. This has been described because it's more a natural state of them being, not a specific action as used with で in other examples. But because the fish is swimming slowly, I think this is no longer the natural state and now becomes a different action, which gives the justification for で. Please correct me if I'm wrong, I'm still working out the difference between these particles.
First it's the boy is performing the action of swimming in a river so it's で then fish always exist in a river so it's technically に because reasons and now its で for reasons unknown to even possibly duo.
Is there maybe, I don't know, a different river I was supposed to be aware of before attempting this sentence that makes it obvious it is で and not に or is it just that obvious the more you learn the language. Probably the most inconsistent unintuitive particle I have had to deal with yet... So frustrating.
It has nothing to do with the speed of the fish, but what is happening at the location. で is used for the location at which an action occurs, not necessarily any verb. に is used for the target of a verb or action, and also the location of an existence verb, such as "to exist." In all of the instances mentioned, Duolingo is being consistent, it's just hard to see unless you take a look at some of the comments along the way.
Probably because it's very unnecessary. It's like saying "inside of the river" but even more, and is just weird. 川で clearly means that you are swimming inside, so I'm pretty sure that 川の中 would never be used. You can try reporting it, and maybe it's already accept now, but I wouldn't be too sure.
The て form is for continuous actions, which this in fact is. 泳ぎます is "swim(s)/will swim", 泳ぎました is "did swim/swam." 泳いでいます is "is/am/are swimming", and 泳いでいました is "was/were swimming."
If we didn't have the て form here, and the sentence was「魚が池でゆっくり泳ぎました」、then it would translate to "the fish slowly swam in the pond", rather than with the て form, "the fish was slowly swimming in the pond."