Grass is uncountable, so you treat it as "it".
"Nice grass! It's so cute!" (I've never heard of anyone calling grass "cute", by the way.)
Correction: "Both seem a bit off." (Both = 2 = they.) You can, however, use "each one" if you want to use the 3rd person "seems": "Each one seems a bit off."
Also, "fish" can be singular, plural or uncountable depending on the context. It's probably not the best example to use here.