Confusing Conundrum

[nortonmad213]

recently i have lost 3 of my four leopard danio's.
i have tested my levels and they come out at 0,0,3.
i have done water changes of around 50% every other day to try and control it.
all other fish in the tank seem to be fine, swimming around rapid as hell, eating as hungrily as ever,
i have only ever had trouble with leopard danios,
could it be that they all had a genetic defect? (i bought them all at the same shop at the same time)
or am i over looking something?
thanks in advance.
sorry for length

 

[DerpPH]

What is the current stock? Genetic defects do rarely occur. even in danios. well what do you really mean as in rapid as hell? what size is your tank? 
 
Sorry if there are many questions. 

 

[nortonmad213]

sorry for the vague explanation.
i have now 1 leopard danio and 4 zebra danios, 2 gecko loaches(i think), and four cherry shrimp,
they are in a 30 gallon tank, they zip really quite fast.
no worries the more info i can give you the more chance of help lol