Got a shop vac?
Only idea I had other than that was to tape something that you could try to slide under Mr. Fishy, a garden trowel maybe...
As for the grate, I'd suggest more weight. A few small metal weights tied to the grate with monofiliment and dropped inside the tank (hanging just shy of the bottom) should prevent Mr. Fishy Jr. from knocking the grate off, while still allowing you to use said grate as a topper (and wouldn't be as visually offensive as using duct tape to hold it down).
Into the fires of forever, we will fly through the heavens, With the power of the universe we stand strong together
Through the forcing of power, we will soon reach the hour, For victory we ride, Fury of the Storm!