A fry box has to have pristine water. I prefer the ones that hang on the inside of a tank and use the tank water, so it's filtered and circulating all the time. Keep the bottom bare, and if at all possible, feed live microworms. Ideal fry food and they live for many days, so the fry will pick them off the bottom and you won't have to feed five or six times daily.
Otherwise, you need to feed several times daily and keep water very, very clean. Fry can be overwhelmed by ammonia or nitrite or lack of oxygen much faster than adult fish. Stagnation happens when there is no circulation of water, so the gas exchange stops happening.