Problem with aquarium?

Hi all..its serious matter plz help me.I am from Bhutan,bt m studying medicine and surgery here in bangladesh and few weeks back i Bought a Fish Bowl,and had four fishes in it,2 gold fish and 2 black mole..bt later i got another tank 18' long tank and i put those 4 fishes from the bowl to the tank..as the bowl was empty i bought 2 small gold fishes there.bt one died the next day and i thought the other must be feeling lonely so put it to the tank bt it also died dan the 2 gold fish also died and now the black mole.so i bought a sweeper fish and a chines fish,and two lil ones bt all died and now m left with the chinese fish,a black mole and sweeper,,each day around 2 fishes diess i dont know why day by day all the dishes are dying help me plz...:'(