Thanks I appreciate it. The one thing that you mentioned that would concern me is quarter-inch deflection. For a floor it's no problem because it flexes. The fish tank doesn't so I would keep an eye on that as I would think that would put stress on the tank possibly leading to a leak. I would keep an eye on that over time.
I am not overly concerned. No doubt I will be nervous during the fill, but I think I have mitigated the risk.
First, I have margin. The beam were designed for 5K lbs load, 2500 lbs each, evenly distributed on basically half the beam (6ft tank 13ft span). My final tank size is 72x36x26 exterior dimensions, so internal volume on the order of 71x35x25, assume a fill level 1" below that, so 71x35x24=258 gallons, round up, and that puts the water around 2200lbs, assume 500 for the tank itself, 500 for the stand and equipment, a couple hundred lbs of rock and stuff, final weight around 3500 lbs. The beams are in addition to the existing floor structure, not instead of.
Second, I'd be willing to bet there are thousands of tanks out there that are slightly out of level. Not that I recommend it, but I have no doubt it happens a lot!! Unless you are putting the tank on a concrete slab, the floor will move, its just a matter of how much. I'd bet 1/4 inch over 6ft doesn't seem like it would be much of an issue. I would think that flatness of the stand would be move significant...that would really stress the joints.
Third, I had leveling feet added to the stand. After plumbing I'll do a freshwater fill/leak test. During that time I can monitor the sag of the floor and adjust the leveling feet as necessary. Besides the aforementioned laser measuring of the height in under the beams, I'll stick my 6ft level on the tank while we go!
If I have missed something, please do let me know...it would be an awfully expensive mistake to make!