I don’t understand the symbolism

Those are betta fishes. These fishes cannot survive together in the same tank especially the male ones because they're very territorial and potentially kill each other. I think it is depiction to show how their ideologies vary with each other and clashed and how they could no longer be in peace with each other. Also, Gojo is represented with a light colored healthy fish while Geto's is a darker one with jagged or rotten edged fins representing his declining mental health