What species of lion, how long have you had him, does all your water quality parameters check out, has anything changed in your tank like new inhabitants.
Is it a volitan lion or one of the dwarf or medium bodied lions. Volitans take better to feeding dead food. Sometimes the others may decide not to eat dead food. If it's a volitan does he seem healthy otherwise, how does his color look, has his activity changed at all. Live food like ghost shrimp usually triggers a feeding response, if not then there's some other underlying issue. Four days for a lion really isn't that long, how often do you usually feed him. Every other day to every third day until you see a slight bulge in his belly is better than every day. In the wild they would gorge and fast.
I consider 80 very high, it is for my fowlrs. My angler tank is usually last on my maintenance list. His appetite waned and I found nitrates to be over 30. I did a series of water changes to get it down to 10. His appetite almost immediately rebounded. Every fish will react differently to different levels of nitrates, it's just something to consider.