It could be a few things:
A) She is getting bullied badly by the fish and is getting stressed - which always ends in death.
B) The dalmation has had her fins ripped causing her to swim oddly. (any signs?)
C) Swim bladder disease.
I suggest seperating them NOW, before it's too late, as for swim bladder disease, it is suggested on feeding the molly peas, as it helps with their digestion.. and can cure swim bladder disease, otherwise your fish will die.. seperate them NOW. lol, or else try baths in Epsom salts (70g / litre for 5 minutes) which has the same effect.
In my dreams Iím dying all the time, As I wake itís kaleidoscopic mind, I never meant to hurt you, I never meant to lie, So this is goodbye, This is goodbye, Tell the truth you never wanted me, Tell me, In my dreams Iím jealous all the time, As I wake Iím going out of my mind, Going out of my mind... ♥