From the care guide:
Salamander Norns are strictly carnivorous, and plants and fruits have no nutritional value to them whatsoever. In order to keep them healthy, you should feed your Salamander Norns bugs and critters as a source of protein, although you should also feed them food, such as carrots or cheese, in addition to this, in order to provide them with fat. Salamander Norns are also detritivores, and must eat detritus as it provides them with starch. Keep in mind that Salamander Norns are not Toxic Norns, and they can still die if they become heavily infected with bacteria.
As there aren't any edible critters onboard the Capillata, you should try downloading some, or you could use the Crobsters from the Bondi Norn Pack. You could also try feeding your Salamander Norns stingers as they are immune to Geddonase, but they are still somewhat painful to eat. Many critters and bugs from the Creatures 3 Ark, such as dragonfly larvae and hedgehogs, are also good food sources.
Get them here.