That looks awesome, I dig the scape too.
I know that you mentioned that Elos had done finite element analysis on the stand, but I would double-ask them to view the results. Doing deflection analysis of those horizontal members and buckling analysis of the verticals with that much weight would be illuminating.
I just ran this analysis for 5000lbs mass and ended up settling on 2"x3" tube for the top ring and 2"x2" vertical supports which resulted in negligible deflection. Your water, plus rock, plus tank, likely weighs closer to 6,000lbs, and then add a 20% safety factor, and I would be concerned with that structure.
Anyhow if they did the analysis they likely saved the model and could provide you screenshots of the deflection with no effort.
I hope that it will all be fine, and I know that Elos knows their stuff, I just want to help ensure your long term success!