Nine fish can swim freely with this?

• A chamber work by Louis Spohr was the first to bear this title
• The planets of the Solar System, at one point ('90s kids remember)
• Multipart opus
• Musical group
• Sign of confidence in acrobat group?
• Nine singers singing
• Largish singing group
• Music for nine instruments or voices

Wikipedia Word definitions in Wikipedia
Franz Lachners Nonet in F major is a composition for chamber ensamble published in 1875.