Regarding to food it looks all right. It is strange that he eats algae wafer though. Apisto's do need carnivorous foods. This, with its old age won't do any good. SO try to get him to eat more carnivorous foods, and a lot of them to get him fatter.
The other fish do not look very skinny. So I would suggest keep on feeding frozen foods and hope the medication will do the trick this time.
However, looking at the other foods, frozen carnivorous dried foods, this shouldn't be a problem.
Watch out with bloodworms. Apisto's don't digest them very well. Besides that do these larvae have tiny hooks. These hooks can cause constipation, especially if your fish is not very healthy. So try to avoid these.
Regarding to your question about the daphnia, that is a little fly. These are harmless and come very often with dried or frozen daphnia. So you could find a lot more of them.