He was so stressed from the move that he plucked one of his claws off, which is something i've been kind of anticipating to happen. I'm definitely on high-alert with him now.
Yeah, that's definitely not a good sign.
You mentioned lighting and i will admit that his lighting has been fairly irregular. Lately i will have the LED lights on at night and then have the light off and the aquarium covered so it's moderately dark during the day, but not completely.
That might be adding to the stress.
I should mention that during the move 8 months ago, he was inside the 40 gallon when we had an accident, resulting in the tank slamming to the concrete and shattering (and cutting my lateral artery in the process, sending me to the hospital). Sebastian was rescued by an EMT who brought him up to my room and put him in a room-temp bucket of salt water (removed from his tank earlier for moving). He would spend about 3 weeks in that Home Depot bucket (with the chiller and a small filter), he actually managed to escape the bucket and i found him on my carpet once. The reason for this situation is my left hand was completely unusable. He then went to the 10-gallon for 7 months and remains there today (only because nobody will help me with moving in my new 40-gallon lol). Then i moved him again a couple days ago where he spent about 1.5 hours in a Home Depot bucket again with no aeration and the water allowed to warm to 10°F above normal for him before i had his tank setup (again, nobody helped me set up his new tank in advance so i had to just move his current tank and set it up quickly). This is when he dropped his claw. So he has been put under a lot of stress just from all this drama.
Wow - glad you're alright; that would definitely be highly stressful for the crab.
Sebastian is 5" in carapace width.
Yeah, a 10 gallon is definitely too small for a crab that large.
If i could critique my own habits, water changes would be an area i need to improve. The ammonia levels are probably fairly high, especially while i was injured (basically no cleaning of the water at all for two months).
Yeah, with a 5" crab in a 10 gallon tank, I'd definitely get some ammonia tests and see how it's doing ammonia-wise; too high of ammonia can kill critters (slowly if only somewhat too high, and quickly if much too high).
I use Aqueon water conditioner in tap water. Not sure of any specifics beyond that, i'm pretty novice.
So, the water conditioner is designed to remove chlorine/chloramines from the water - that's good, but it doesn't prevent other issues that tap water can potentially have (such as excess metals like copper, which are toxic to inverts).
Seattle's water quality report is pretty lackluster compared to some I've seen, but it does list it's copper levels, so we have a decent example to work with here: 90% of houses had copper levels at or below 0.12 ppm.
Seems like a small amount, right? However, when you look at the copper level tolerances of various inverts (including, reportedly, corals), you find:
0.02 ppm can kill some inverts (reportedly including corals) - in fact, 0.002 ppm (A.K.A. 2 ppb) is enough to kill certain inverts.
Unfortunately, Seattle doesn't list the minimum levels of copper found, but it still means there's a chance that your crab could be getting slowly poisoned by the tap water - I'm not saying that's the case here (particularly with your previous crab having done fine, though different species have different tolerance levels - sometimes drastically different), but it's a possibility: this is why I personally always recommend RO/DI water.
Anyway, moving on to address a couple of other points that may help:
There is nothing but sand from Petco in the current tank (for 8 months). In his previous 40-gal home, he had rocks from the ocean with various other types of critters (sea anemone, eel grass, clams, etc) which he lived with in the Sound.
I would definitely add something to the tank to give the crab a place to hide, even if it's just a plastic overhang to hide under or a large PVC tube it can hide in.
In addition to decreasing stress by giving the crab a way to seek shelter, this also provides more surface area for more nitrifying bacteria, which could help with any excess ammonia levels.
And, finally:
Mostly i have been feeding him Tetra PlecoWafers, about once per day, and giving him a piece of fish or shrimp once a week or so. I think it is erring on the low end of feeding frequency but it's because the 10-gal gets dirty very fast and he is not always hungry, resulting in wafers sitting around making a mess. He used to get extremely aggressive when he smelled meat, now he is kind of lethargic but still interested. I fed him tilapia and spotted prawns (from the Puget Sound). Back in the day, he actually managed to kill and eat a live one that i had coexisting with him.
My previous crab (Graceful Crab) which molted just fine had a diet of mostly Crab Cuisine, which are too small for this bigger crab.
I suspect the Tetra Pleco Wafers just aren't cutting it as a diet staple, especially if only supplemented once a week with meaty foods; it's low in protein, low in fat, likely high in carbs, and essentially a vegan/vegetarian diet for an omnivorous (primarily carnivorous) crab.
Similarly, tilapia is also pretty low in protein and fat.
For a staple diet, I'd suggest something like
Mytilus sp. mussels as the main feed, supplemented probably every daily or every other day with a smaller portion of NLS Marine Fish Pellets. I'd also suggest supplementing two or three times a week with a good quality fish (such as salmon), shrimp or other crustaceans (like mysis), and something like fresh urchins or snails if possible.
That said, the mussels would be by far the most important part, and the others would honestly just be nice additions to try and add variety with to ensure a well-balanced nutrition and optimal long-term health for the crab.
As an alternative to the NLS pellets, you can use macroalgae like Nori, Ulva, Halymenia, etc. (possibly even some kind of kelp) - the crab might eat it by itself, or you could try things like wrapping it around the meaty food, making a gelatin feed with it, etc.
