Are the tanks resting on the green horizontal joists? Or are they sitting on the red beams? I can't tell. Whatever is actually holding up the weight of the tanks is what really needs a vertical wood column (ie, post) to directly support it.
This design is very close, but it makes me wonder. It holds tons of weight. I see the top long red beams have yellow vertical 2x posts directly underneath them, which is good. But the middle tier red beams seem to be missing those supporting posts beneath them. Or am I missing something? Posts must do the lifting. The beams only transfer weight to the posts. Long span beams will flex, which is scary. Hence building codes will limit the span of beams. 12 feet is super long. If the weight actually rests on the red beams, not on the green "joists", then you'd need vertical columns (eg, notched 4x4 posts) to hold up the beams, not the green joists. That's the red flag. So is the lack of diagonal bracing, to prevent "racking" collapse. Just a few diagonal 2x4s is enough. Also, I'd suggest horizontal 2x4s near the top to firmly anchor it to a solid wall (or to ceiling joists), so it doesn't topple over onto someone.
We can compare this to building a small non-ledger deck (they're very similar). Someone stated that none of the weight should be borne by fasteners, only by upright wood columns. Yes! Fasteners are there only to hold the structure together rigidly, not to support the weight of anything. Fasteners fail catastrophically. Solid columns/posts do not. So house decks require notched posts (not bolts) to hold up the load-carrying members (ie, beams or joists), same as this fish rack. A 12-foot 2x4 beam is gonna be scary. You can sister them, or use 4x4 beams.
It's gonna look epic when it's done. I look fwd to seeing all your happy fishes in their spiffy new homes.