３ヵ月後

陸上に置くと吐くし水槽はこれしかない

So, this fanon set the aquatic Marisa to not be able to stand above ground.

guessing at one point they were small and the whole aquarium was their world but now they've grown to such a size that they're cramped and there's only a dribble of water

or

mr owner just threw them in there with a cup of water and said, there ya go.

They can't get out either.
Once put outside water, they start vomitting and would eventually die.