I've always been told:
1. "what your fish can eat in 3 or 4 minutes is enough"
2. "Slightly hungry fish are better than slightly overfed ones"
We tried 1 cube that was WAY too much.
We just tried 1/2 cube, and although it was far better, it was still too much food. There were a some pieces floating about for about 10 minutes.
I'm thinking we'll try 1/3 of a cube next until we add some more fish.
All in all I'm pretty happy with the way the tank is running. After our first 24 hours with the fish, the Ammonia spiked up a bit, but it's almost 100% gone now, so the tank is definitely reacting to the bioload properly.
Nitrites are currently sitting right around .75 ppm
Nitrates are basically non-existent.
We will keep you all posted.