How Much Light Do I Need?

[RinaLane]

I need just 1? And it will be enough? I guess I can turn only 1 on then?

 

[Byron]

I need just 1? And it will be enough? I guess I can turn only 1 on then?

 
It depends upon the fixture, but most only allow both tubes to light.  Removing one tube will break the circuit and the other won't light.  Unless the fixture is wired to allow one tube only.
 
Believe me, T5 is bright light.  I had two tubes over my five-foot 115g tank, and I jokingly said my poor fish would be petitioning me for sunglasses if I left it.  Needless to say, I didn't; they are now much happier.

 

[RinaLane]

Now you scaring me away from t5 with algae grow & sunlight brightness

 

[Byron]

Now you scaring me away from t5 with algae grow & sunlight brightness

 
Better safe than sorry.  Aside from algae (and I can guarantee you will have issues with bright light) light affects fish considerably.  I always have floating plants in my tanks partly for this reason.  Most fish we keep do not come from open water exposed to sunlight but from water shaded by forest canopy.  Discus for instance will not appreciate bright light overhead.

 

[RinaLane]

Thank you !