Try a slice of cucumber or courgette, butternut squash, red pepper... Sent from my iPhone using Tapatalk
Are they constantly staying at one place ot do you see them move around the tank?
Id keep trying with veg whether you see them at it some take a while to realise its food. Mine eat veg seen them eat catfish pellets but rarely see them eating any blood worm etcSent from my iPhone using Tapatalk