Are they even big enough? I think clowns reach sexual maturity at about 15cm/6inch. So far that's taken mine up to 8 years (I've had the oldest since may 2006, so it is possibly 10+ I think because it was a good size at purchase maybe 3-4 inch)
They grow quite fast at a young age, past 4inch, they slow down loads! Maybe 1/2" a year.
My biggest is at or around 17cm about 7 1/2"
https://www.flickr.com/photos/92891884@N04/13254225313/
(Tank bred clowns is a lucky thing indeed, very few manage it naturally, most of the actual activity is speculation really with a few varied ideas of how when and why they do the dance)