接続数かあ。ってあれ、ブラウザ1枚開いてて6本は少なすぎないかい？
$ heroku pg:info -a zundan-mastodon
=== DATABASE_URL, PG_HOBBY_BASIC_URL
Plan: Hobby-basic
Status: Available
Connections: 6/20
PG Version: 9.6.1
Created: 2017-04-20 18:31 UTC
Data Size: 95.9 MB
Tables: 29
Rows: 140465/10000000 (In compliance)
Fork/Follow: Unsupported
Rollback: Unsupported
Add-on: postgresql-aerodynamic-17416