It's fairly easy to build/retrofit a top..
Thanks to tiny-ness of LED's..
The normal 120 degree spread means lights don't need to be too far off the water line..
aesthetics are 1/2 of the equation afaic.....
You could start w 1WPG if you so choose. CRUDE but turns out to be fairly effective till you get a better handle on things.
errors on the high side usually..
biggest problem (and actually larger than the "old" tube wpg rule) is lumens/watt output of LEd's can be sig. different from one diode to another
Sometimes 3x different.. ie. really cheap Chinese vs premium CREE's
Added point is if the light is 3x less it may not actually be cheaper.. on an output basis. Now how much output one needs or wants is ???
Sort of a "standard" light..
roughly 100Lumens/watt about the same as your t8's ..slightly stronger.. better delivery..
Delivery accounts for an almost 2x increase in effective output..though a narrower field.. which can be mildly problematic for wider tanks >16" ish
48 par 5200 lumens.. 60 theoretical watts..