This is the 125Ltr tank when I first bought it and it now has 2.5WPG. Recognise it?
Here is a pic after the upgrade
And a daylight version so you can see for sure.
Admittedly it is the Duo version. Fluval Duo Deep 800.
Can you take a picture of the inside of the hood?
I personally think that you should be able to rip the light and fitting you currently have out and screw in a power compact in its place. maybe 2 lights if you remove the ballast recess that the current lights use.
You cut out most of the ballast recess leaveing the upper bit where the panel on top clips into.
Then you screw 2 power compacts into the firstly the part where the old T8 light was then one into where the panels is like this.
It will fit 2 36W PC lamps no problems with 2 reflectors. (72W)
Only problem I can see now is that being 14 years old you would need to find at least Â£55 to do this and if you wanted each light to switch independently then you would need 2 starters so even more.