Hi. I am fishless cycling a 125 gallon tank. It was started using a cycled sponge filter and a cycled 2nd sponge in my new AC 110 filter. I'm about 4 weeks in. The ammonia is 0 ppm and nitrite is at about 2 ppm in 24 hours. The tank is on the 5th ammonia dose. Yesterday I noticed the airline had come loose from the sponge filter. Today I noticed that an area of sand has some copper-brown areas. What could this be? Any help would be appreciated! Thanks!