I have to say, white sand shows everything; poo, plant & wood debris, vacuum more. Your cory probably died from the ammonia during cycling (as Bob said), it can take a while to actually die from ammonia burns. The foam is probably from protiens in fish food etc, not a big deal, use a paper towel to lift it off the surface. An AC 50 is minimal filtration IMO, add a second filter or keep animal life to a minimum.