[Kathyy]

If you could hang the light a PAR LED bulb might work out for a cube tank. Might have to be a powerful one hung high to get the coverage you need though. You would need 2 strips to cover the width of the tank so it might be cheaper to go with a single pendant.

Seachem fertz once a week probably isn't enough for a high tech tank though. And are you putting in all the CO2 you can? How long are you running the lights each day?

See how much you are actually dosing using this calculator.
http://calc.petalphile.com/

Without CO2 or much dosing I did have ground covers not dying in a 20" tall tank with 6 T12 bulbs over it. Probably running a 12 hour day back then. I suspect the ground covers would have done better had I known about CO2 and complete fertilizing.

 

[senior el roboto]

The lights are on for about13 hrs a day. Yeah enough co2 the keep it in the norm. The bottle only says once to twice a week on adding ferts.
The lighting will have to be hung inside the canopy. It comes off the top of the tank about 10-12"