You can reference a tag in a post, but only a topic can actually be tagged as such.
However, if you have the data explorer plugin you could use a query something like this to pull up a list of ones where the tag has been linked in a post:
-- [params]
-- string :hashtag
-- date :start_date
-- date :end_date
WITH target_posts AS (
SELECT
p.id AS post_id,
t.category_id,
p.created_at
FROM posts p
JOIN topics t ON p.topic_id = t.id
JOIN users u ON u.id = p.user_id
WHERE t.deleted_at IS NULL
AND t.archetype = 'regular'
AND p.deleted_at IS NULL
AND p.post_type = 1
AND p.created_at::date BETWEEN :start_date AND :end_date
AND p.post_number <> 1
)
SELECT
tp.category_id,
tp.post_id,
tp.created_at::date
FROM target_posts tp
LEFT JOIN post_search_data psd ON psd.post_id = tp.post_id
WHERE psd.search_data @@ TO_TSQUERY(:hashtag)
ORDER BY tp.created_at DESC
