Hello. New to salt water, not to the aquarium hobby. This will be a little long. About a month ago we decided to buy a saltwater tank. So, for a few weeks I studied everything I could so we could make a good decision. Two weeks ago this Saturday, we bought a used 180g (signature). The tank was up and running with fish in it. It had been in use for 4 years. We wanted to try and save the fish so the first thing we did was go to Petco and buy a 14g tank and set it up at home. We then went back and got the fish, a 3-striped damsel, cinnamon clown, yellow tail damsel, some type of large anemone, and a big snail. We carried the fish, some live rock, and 15 gallons of the good water home and got them all set up. They are doing well in the 14g. Stay with me. I put 3 pieces of live rock in water also to try and save it. We emptied the 180g, and got it home. The next day, I cleaned the 180g with fresh water with the sand in it. We got enough help to get the tank set up in the house. We put all the rock that I didn't try to save back in and filled it with salt water. The next day I put the three pieces of live rock in that I kept in water overnight. At that point the water tested 1.022 and everything else was great, temp 79. After a week to get the water adjusted, we put in 5 green chromis to start the cycle. Levels have been good everyday, ph 8.0, no ammonia, salt 1.022, nitrite and nitrate near 0. I keep it on a spreadsheet. This morning the ammonia was .5 and the nitrate was 10. The water is crystal clear. This afternoon I noticed the brown algae is forming on the sand and rock. The ammonia is back at near 0 and I never saw any nitrite. All this and my question is, is it possible the tank has already cycled? Could the rock and sand have had enough bacteria survive to cycle the tank that quick. Any suggestion? I am not planning to add any fish for about 6 weeks, but I didn't expect the brown algae until later in the cycle. Thanks in advance.