It would need to be a really small light. Sometimes metal halides give a shimmer, and you can do it with a CFL if the fixture is far enough away from the tank.
Wait do you actually want LED's or are you just saying you want it to look like an LED?
Looks like he wants to replace the t's w/ cannons" or par cans.....
To the O/p ... You can think on the lines of 1/2W LED for every 1W of T8's..IF your staying at about the same height as the orig lights.
Then you need to decide on spread (spot or flood) and coverage both on tank and between lights..
It is a bit complicated..
If you set the lights low.. floods work best and farther spacing .. higher more "spot"..tighter spacing..
The can size is less important than the watts/beam angle..
Of course the bigger the can the more LED's they tend to pack into it..on a commercial basis.. but you can pack 100W of LEd's in a par 20 space. but they don't usually (more like 7)
And you have to be careful w/ large par lights w/ large Watt density.. Tend to overheat and burn out..Reports have failures going as high as 100% over a year or 2. (internet scuttlebut)
Oh and most PAR lights certainly aren't targeted for plant growth..w/ the exception of purple looking "plant lights" and overpriced "reef" lights (more blue) .. Fortunately LED's are natively high PAR.. but to be more balanced look to the lower K ones (4000ish)
Keep in mind they are naturally a bit "warm" in color...
Only "data" I've found and it is incomplete:
60° optics = 12" square spread, reaching 2ft deep
90°optics = 14' square spread, reaching 1.3ft deep
T5 to LED Comparison
*(2) 18 Watt T-5 Dual Fixture = $60
*(2) 18 Watt T-5 Bulb = $30
(it takes two T5 to equal one AquaBeam 600 12 watt in actual useful light energy)