全体的に、非常に分かりやすく、まとまった英文だとの印象を受けます。
若干の修正をすれば、更に優れた内容になると思います。
参考にしてください。

1. The municipal government was troubled by the townspeople throwing garbage into the river, so they discussed the problem and let colorful carp go.
→ The municipal/local government was troubled by the townspeople throwing garbage into the river, so they discussed the problem and stocked the river with colored carp.
※let go …：…(手でつかんでいるもの)を手放す／stock the river with colored carp 川に錦鯉を放流する　錦鯉：colorful(色彩豊かな)/colored(色のついた) carp のどちらでも良い　

2. Then, the number of people throwing garbage dropped dramatically and it became a clean river six month later.
→ Then, it worked beautifully(それは見事にうまく行った。）; the number of people throwing garbage dropped dramatically and the river became clean six month later.
※it became a clean river:itの中身が不明 → the river became clean

3. That's why other local governments seem to be rushing to inquire. 〇
→ As a result other local governments seem to be rushing to inquire about the town's approach.
結果として、他の地自体がこの町の取り組みについての問い合わせが殺到しているようである。

以上