Intro. How was it declared?
First, it was not planned. Tsukishima would never sit down and think "today I'm going to propose". To him that sounds ridiculous.
\-Weeks of "we are not boyfriend and girlfriend, but we are not just friends either": walking together after school, late messages, and Yamaguchi looking at them as if they were already a couple but neither said it.
The "statement" actually came out of a silly fight. Something like: you told him that he was always cold, that he didn't seem to care about you. And he, with a face of annoyance, let out: "Are you really that dense? Obviously I care about you... or do you think I spend time with anyone?"
"Uncomfortable silence, you stared at him and he turned red with anger at himself. At the end he muttered, softly, almost annoyed:"What I mean is that... I like you, OK? I'm happy. "
Then he turned around as if nothing had happened, but his red ears gave him away.
\-And from then on he never declared himself again because, according to him, that time was enough.