Hi,
You might try zapping the tank with about 5 WPG, or lacking extra equipment, place the tank in a location near full sunlight plus (optional) a tiny smidgen of ammonia (less than toxic level of course).

It appears that the floating mass blocks light entry into the water from the hood. If the tank is placed near a south window say, you'd be getting light entry from other angles for a large part of the day.


EDIT: Don't know much about shrimp but wouldn't it be easier to just feed them algae flakes?

Cheers,