"さかなが川でゆっくりおよいでいました。"

Translation: The fish was slowly swimming in the river.

[Doubledealer]

So I guess we can't put "The fish swam slowly in the river" because we need to put 'swimming' due to the ”でいました”？

[KeithWong9]

泳ぎました = swam

泳いでいました = was swimming