Emaciated Zebra Danio

Morning all. My tank has been established for 5 months with fish in for the last 3. Last week I noticed one of the zebra danios looking very thin and within a couple of hours he was dead. Prior to the last couple of weeks everything seemed fine and the fish seemed to be growing ok but now I just have a feeling that something is wrong in the tank. Ammonia & nitrite have always been 0 and nitrate which starts at 40 in our tap water never goes above 60/70. I do a 50% change every week. Could it be that I'm not feeding enough they seem to spit out the flakes and micro pellets but eat the frozen shrimp and bloodworms I give once a week ok.

i had exactly the same problem with a zebra danio i had 12 and after about 6 months of having them i noticed one got very two got very thin and week i found one of the two floating dead in a bush a few days later and the second strugling by my filter intake so i thort he was done for two but that was almost 2 and half months ago now and the second one picked up and is not healthy and fat again swimming happily with the other 10 danio's i never did find out what caused it exactly only thing i did add was a dose of melafix and pimafix for a few days.