To add to what Minh said, the gilled-ones can not resist live food, like brine shrimp. The problem is that brine shrimp are not very nutritious.
I recommend that you remove the water from a brine shrimp portion by pouring the contents thru a net. Discard the water and place the remaining brine shrimp in a container. Add some Selcon to the brine shrimp and let them soak for 20 to 30 minutes or so. The longer you wait, the more the brine shrimp will absorb the suppliment. With that said, however, don't let the brine shrimp soak for days on end. Add the brine shrimp to your tank and watch the gilled-ones go crazy. The gut-loaded brine shrimp will provide more nutrition.
After a meal or 2 of those gut-loaded brine shrimp, your hippo should be doing much better.