I drop 4 frozen cubes in a ketchup bottle with warm water and shake until the all break up. I then use that bottle to feed two tanks, for however many days it takes to empty it. I then refill it.
Been doing this for months now, no issues.
If you think about it...milk sits in the fridge for weeks. So does fruit, yogurt, all manner of other stuff that you eat. Why can't fish food last more than a day or two?