ok last nite I purchased a new filter, and today apon installing it I had to move my skimmer. well behind the skimmer sitting on the top were 2 crabs almost looking like spiders I thought were livng on it until I looked and they were just sitting above the water since I lowered the water level. They were blackish greyish. with white dots. bout the size of a quarter. I didnt want to touch them worrying they would be fragile. Now thats exciting seeing something new that I didnt put in there myself. but very confusing when I purchased a cleaner shrimp or a cbs they both died within 24 hours. Now at first I thought they were bad from the store. then I remembered they were in the store for a few weeks thriving very well. Im so confused by this hobby its so frustrating one minute when I say I give up Im selling everything. Then the next I see those crabs or tiny clams in my live rock I get excited for it all over again!