Large internode length and stretching up are typical signs
of insufficient light.

My tank is 3 x 2 x 2 ft.
I turn on my CO2 supply, and 4 tubes of 40W florescent tubes for 7-8 hrs
daily, and apply liquid fertiliser weekly.

160W for an 80gal should be fine, except for very light-demaning plants. I
suspect
that your problem may be the short lighting period. 7-8 hours is pushing it.
Kasselmann
recommends a minimum of 10 hours, and suggests 12-13 hours.

I'd give that a try for a few weeks before changing anything else.

Cheers,

Michi.