Things that crawl out of our rock

The 125 gallon tank (the future Bright Reef) has been cycling mildly, with the ammonia never going above 0.50 and the nitrites never exceeding 0,25 ppm. Yesterday the ammonia was barely measurable, and today we have none. Today the nitrite level is somewhere between 0,25 and 0 ppm, with nitrates at 5 ppm.
But last night, I found a snail, a tiny and very busy snail, zigzagging his way across the glass. I moved the rock around yesterday, fastening some together with zip ties, and that stirred ...

...