move tamaki into the 5gal, 1.5 is really way too small. do not put a halfmoon into the 1.5 either. due to their excessive finnage they are very prone to tail biting when stressed, and being in such a small environment would definitely stress him out. really the only betta that would be all right in it is a baby, and that would only be as a "growing up" tank.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)