basically the deeper the water the more light you need to penetrate it. In a shallow tank of 12inches high that amount of light would be a massive amount. In a tank that is 18inches high it would be heaps, but in a 2ft high tank it will be a reasonably good amount. If the tank was 3ft high then it wouldn't be much at all. Fluoros are great for shallow water (up to about 18inches) but not so good for deeper water. They just don't have the intensity to be able to penetrate deep water. T5 fluoros can do 2ft of water but that is pretty close to their limit. They will be fine on that tank tho. And if you find it is too much light (unlikely but possible) then you can simply turn a couple off.
Have you thought about getting a couple of 150watt metal halides instead of the T5s? They would use fractionally (only a tiny bit) less power but would be ample for that tank.