Here is a nice example of a 100cm tank lit by two T5s approx 40cm above the water, light duration is 6 hours a day.
The tank belongs to Mark Evans and just made Rank 137 IAPLC entry.
We have lots of similar tanks on our forum using 2 x T5 or LED..
Here is an example of what you are not "supposed to" do(38 cm cube):
That is a 70W HQI metal halide about 45 cm from the substrate on for 1 + 7 hrs a day. I run NO CO2, for more than a year the only time I had algae is when I started using tap water-stopped using tap water and the algae is not growing any more. The only ferts are Osmocoat + in the substrate (fluval shrimp stratum). Some plants are museum specimens in my tank (erios) and sure I could grow the plants 3x faster (Yay for pruning and replanting every week), but it is really nice to look at, and many plants such as chain swords, swords, mosses, rotala looking stuff, are doing great.