I think you'll be perfectly fine, I've setup a few tanks on tile before in similar situations and never had an issue. As long as the tile was installed correctly and it's not a delicate natural stone you shouldn't have any issues. With tile your subfloor has to be at least 1.25" thick so it's strong enough to resist any deflection from that kind of load. If the tank is about 16 square feet that's only around 60lbs per sq foot, just be sure the stand distributes the load evenly (foam would be a good idea).
If you're that worried about it bridge some small beams perpendicular between the two joists where the fish tank will sit and attach them to both joists and the subfloor. Putting in that yellow section won't help you though, the joists are plenty strong as is and your only concern would be the small sag from the tank sitting between them but like I said with that subfloor it's a non issue.
Last edited by sphelps; 12-06-2011 at 05:49 AM.
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