My bro just got a hippo, and it wouldn't eat anything I put in the tank (he was on vacation), except the frozen formula two. He loved that stuff. He will prolly eat more things later, but for now that was all I seen him eat. And just for reference, I tried...
Seaweed Selects
Spiralina soaked frozon brine
Formula 2 flakes
Some other flake food crap that they had...