I think if I am going for more T5 bulbs I'd probably try to sell the 4 bulb fixture and just start over with a 6 bulb fixture.
That would be a bummer because I really just bought this one and I am sure I'd take a huge loss on it by reselling it.
I am still considering that option though.

As an alternative I was just considering a mix of LED and the existing T5s. Then it wouldn't be so weird to have two fixtures of the same type on the tank. I could use what I have and benefit from the advantages of the T5s (color choices) as well as get the advantages of the LEDs (Halide-like sparkle and intensity)

Plus in this case 200W output of LEDs would only cost me $50. That would be super affordable - as long as it works and provides usable light.

My main concern is: do these types of LED lights provide usable light?