You need quality food, as all fish foods are not equal. You also need food specifically intended for upper fish and food intended for lower fish.
Flake and micro pellet foods will provide food for the upper fish (and the pellet will likely appeal to the hoplo catfish too). Sinking pellets/disks/tabs are intended for substrate feeders.
Quality brands are those with good ingredients such as whole fish rather than "meal," plus they do not have unnecessary (sometimes harmful) additives. Omega One and New Life Spectrum are two brands I use, and there are others that members will undoubtedly mention.
Feed sparingly; upper fish foods should be gone in seconds. Sinking foods are intended to remain so substrate fish can slowly consume them; some of these are still being chomped on an hour or more after I add them, depending upon the fish.
Feed once daily; manufacturers all say several times, but only fry need this much food. And it doesn't hurt to miss one or two days a week; fry need more regular feeding, but not mature fish, and missing a day or two will benefit. Never feed before you intend doing work (such as the weekly water change) on the tank. Feed preferably earlier in the day than later, and always no sooner than 45-60 minutes after or before the tank light comes on/foes off. Nocturnal fish can be fed in darkness, but that is different.