Fish overpopulation

Several months ago some guy at the office brought fishes to improve the Feng Shui of our company.  What a crappy superstition.  I really have very little patience with people like this.  Anyway, this is none of the fish fault so now that they're there, we must take care of them.  They cannot feed themselves, so it's up to us now.

At first he'd brought I think eight red fish and one black of the same species as the red ones.  I thought that was a strange combination, but after searching in the Internet, voila, Feng Shui was the reasoning behind.

Soon after, five of those fish died, and yep, you guessed it... following the Feng Shui instructions he didn't bring replacement fish because the logic goes that those fish died because they absorved bad luck that was otherwise going to hit somebody in the office.  So for a long time, we had only four red fish in this rather small fish tank.  I think the size of the tank was good for the number of fish.

Now come Monday morning, and I see that suddenly there're seven new fish plus a tiny shrimp!  Five of the new fish are about the same size as the previous red fish, and I think this time the fish tank is rather overpopulated.  Same amount of water, same amount of aeration... fish shit accumulating more than twice as fast as before.  Oh, and by the time I left the office in the afternoon, I couldn't find the shrimp anymore.  I wonder did the fish eat the shrimp?