Is that 2 t5 bulbs, or 2 t5 fixtures. Most of the time, 2 t5 fixtures means 4 bulbs.
Assuming you give yourself 3" of substrate, you will be around 18 or 19" from the light if its on top of the tanks rim. So, depending on how good the reflectors are, you will receive 10-15 PAR per bulb.
So, for 2 bulbs that will net you 20-30 PAR, good low light.
If thats 4 bulbs, you will be at 40-60 PAR, low-medium light. Add in the direct sunlight and you may be too high with 4 bulbs for a low tech setup. So, you could add some excel and up your fert dosing to fend off algae if it comes to that.