bumblebees eat anything they can fit in their mouths, and as far as i know are omnivorous. ours ate anything left on the bottom including many meaty foods. i do not think that algae would be enough for them; you should probably consider adding shrimp pellets, bloodworms, and maybe some flake
i've had ghost catfish for about a year and they eat mainly flake but also suck up any frozen shrimp or bloodworms they can get ahold of, as well as some spirulina flake at times.
hope this helps; i guess the main message is to feed them something with fish protein, even if it is just flake. use the algae disks sparingly and as supplements