Our fish tank is near the couch, so in the evening is when I look in on it the most. Saw our Blue Crayfish on his back, and thought he was dead! Poke him with a wooden spoon, and his legs moved, so I turned him over and wracked my brain for an answer. I don't know how old he is, we haven't had him long. He's max size, and they only live about 5 years or so from what I've read about them online. I immediately thought ammonia levels, but not having test strips on hand I had to just do a water change and see. Then I noticed the flow from the canister filter was weak. So that got taken apart. It was so bogged down with crud in the hoses, not the filter itself. So flushed that all out. Then gave a thought to oxygen levels... for the past several months I've been keeping the water from the filter below the surface level to reduce noise... part of why I hadn't noticed the reduced flow. Changed everything around, added a flow pipe with holes in it, and started breaking surface water. He was so weak I thought he was a goner for sure. This morning he's still alive, slightly more alert, and had moved to the other side of the tank into his cave. So... progress. Now... crayfish at some point shed their skin for growth. We've had him for about 5 months now and he's never done that. When will they? Do they ever stop shedding skin when they reach 5-6 inches? How frequently are they supposed to do that? Will they go lethargic before it happens? The fish never showed any signs of issues... no gasping at the surface or anything. It's a 70 gallon tank, with 2 7 inch gold fish and 3 1 inch sized Mollies. Usually I'm pretty good about water changes, this is the first time I've forgotten for awhile. I'm thinking it was the water that almost did him in. But this skin shedding thing I don't know anything about. With the gold fish being known to boost ammonia levels, and they crayfish being sensitive to that, I have to stay on it. Also, when I took the filter apart for a rinse, on the sides I saw this strange white/clear and hard substance, laid out almost like criss-crossing paths from a slug... like what you see on concrete sidewalks after slugs have been all over it. But it was HARD, it wouldn't rub off, I had to scrub it off. I've used canister filters below the tank for several years now and have never seen anything like it. I don't have any snails or anything, and they wouldn't be in the filter anyways. It isn't in the tank at all.