What you are observing is the main reason this glass catfish is not a good community fish. You have a group of six, which is essential or it might of its own accord refuse to eat. It also cannot have boisterous tankmates for the same reason; cardinals are not generally considered boisterous, but if they are avid eaters this would qualify.. A thickly-planted aquarium providing some swimming space with a gentle current is essential to calm this very timid fish. Floating plants will significantly assist in keeping this fish less stressed.
It preys on insect larvae and small crustaceans mid-water, not on the substrate like most catfish. Live and frozen artemia (brine shrimp), bloodworms, daphnia; it will usually take prepared foods provided they drift in the gentle current. But active tankmates will obviously discourage this.
Try some frozen daphnia (thawed in a dish of warm water before feeding), using a pipette similar to what post #2 was mentioning. Frozen brine shrimp would do similar though being larger this is more likely to attract the cardinals and frighten the glass catfish. A cloud of daphnia encourages most timid fish; I use this for my pencilfish.
. I just use the two straws attached and put pellets down there. Luckily they do get at it.
