Seems like the Tigrinus might be stressed. What kind of bottom dweller tank mates do u have with him/her? Do you have any decorations or driftwood where the Tigrinus can hide? Also try and raise the temp up to 80ish cuz from experience they seem more active in warmer waters. When I first got my Tig at 7" he/she didn't eat for almost 1 month because of stress. Now he/she is 10" and eats like a pig now but I have to use a long plastic grabber and put the food right in front of it's mouth for him to bite. They are lazy hunters. U can also try feeding at night without the lights on as they come out from hiding.