Let me chime in for a minute, the beam you have is a structural box beam and depending on the span on each side of the beam will determine the concentrated load on that beam, I agree that the beam is most likely adequate for 35 to 40 lbs ft. but in your case your concentrated load is:
350 stand
300 glass
1,100 water
200 misc. (lights, etc...)
2,000 across three rafters
660 lbs per truss
60 x 2 for the rafters on each side
720 lbs ft
tank span 6' =334 lbs. ft.
954 lbs. ft.
Normally where a box beam is used would be a bearing wall. So, I am hoping that their is a foundation below that wall. My immediate concern is going to be the bouncy effect once you fill with water. My suggestion would be to install a stud between the studs you have and possibly another stud wall under the front or rear side (depth) of the tank to have an even distribution on all sides. 120 gallons with Rock and whatever else you decide to fill it with could easily become 3500 lbs and in that case you are taking a big chance.
I hope this helps.
