when you say back fin is it the tail fin or the dorsal fin on the top?
what size tank have you got and any other fish? has the tank been set up long and what type of filter?
do you have any test resultd for ammonia, nitrite, nitrate and ph?
does the tail look like it has been bitten or is it ragged and have any white to it?
it looked as if it was bitten,,and im new at all the is so any tipa wud be a great help....the petshop arent any good they neva tell wot fish are ok with others..they just let u buy them
i dont know if a bite from the top fin would have killed it, so i'm stumped to what happended as the other fish should be peaceful.
how many danios and mollies have you got? how big is your tank?
Do a water change if you have no scaless fish add some salt to the tank, do you have test results in ammonia,nitrite,nitrate,and ph, what size is the tank, how many fish and which type.
i dont know exaually how big the tank is but its got 2 mollies bout 6 danios,4 cherry barbs,some glass.2 pongo eels..i dont know how to test for those,,im doin a full water change tom xxx
Please don't do a full water change it will take you even further back, you do have a filter don't you, take a sample of your water to the lfs and tell them to write the readings down for you,do you have a box or a name of the tank to see how many gallons it holds.