The mystery of what kind of reality this is doesn't seem to matter nearly as much to Tadashi when his kid brother is standing in front him, looking like he's about to cry. Super weird magically appearing aquarium be damned, protective Dashi mode has been activated.

"Hey. Hey, woah." He takes long, quick strides to Hiro's side and crouches over him, then places his hands reassuringly on the boy's shoulders. "Everything's alright. I don't know where we are, but- at least we're both here. It's okay."

Everything else takes a backseat for the moment, Tadashi's attention focused on calm, breathe, fix, heal. He smiles encouragingly at Hiro, and keeps his grip tight, mistakenly of the opinion that reaffirming his presence here will actually help rather than maybe, well. Cause panic.