Springtails, make sure you get the right type. Either temperate for a colder temp enclosure or tropical if a warmer one. They will need a layer of charcoal in the substrate to proliferate and you can buy them online from lots of places.
Their eyes go darker at night time, so it depends if it was dark or not when you noticed this. Are you sure it is still alive? Because a dead mantis also has black eyes.
Yeah ants are bad for the nymphs, I used aphids when I didn't want to deal with / was short on fruit flies. They worked great and they are completely free.