Half my crabs suddenly died

[CONE_MacFlounde]

Sorry yea if I
I just spent about an hour trying to explain what just happened in the best way I could
Site glitched out when I went to post and lost everything I just wrote
Sorry if its not as detailed as yea

Basically asked on reddit yesterday didnt really get an answer so asking here instead

I had to exile a crab a couple days ago
His name is sussy baka I didnt name him
But hes evil just aggressive and mean for no real reason
I set up a 4 gal tank for him with some ugly liverock from the tank and then water from the main tank to hopefully avoid the need to cycle
I know I could rehome him but if hes going to keep being aggressive then rehoming him would only mean getting someone else to euthanise him and personally thats less morally justifiable than doing it myself and well even that feels wrong

Anyway
My tank was low on the salinity level
I yea replaced the 4 gal i took out with water
the replacement water was saltier than the water in the tank
i changed the water slowly doing it 1 gal at a time with a 10min gap between
i mixed the water with my hand and then later the wavemaker too
i poured the water in slowly

the next morning i wake to find what i thought was a moult from the porcelain anemone crab
after looking closer and smelling yea it wasnt a moult
i then got worried about the decorator crab because it was right next to it and i was suprised it didnt react to me at all
after closer inspection didnt look good
he was limp too
his egg flap part was just loose and he didnt react to me at all

then this morning i wake up
one of my crabs had knocked some stuff over
i moved him out of the way and put everything back to where it should be
i know this crab pretty well
his name is loki
he didnt react at all
normally he would
he hasnt moved from where i placed him over an hour ago

theres also a pompom crab
i havent seen it
it was pretty shy before like i only saw it a couple times in the week weve had it

anyway
i can see a similar thing between them
the porcealain crab was well used to hide under my anemone
he no longer does
idk why but he just stopped
the decorator crab and pompom were only recently added
and loki well he was sussy bakas main target
he nearly killed him a while ago and while they had stopped fighting recently loki always seemed shy
so all three(four) were probably already stressed
so im wondering if the salinity change pushed them over the edge

as for the tank
nh3 ~0.1 that just wont go away
no2 0
no3 ~5-10
kh 12
ph 8
27c stable
salinity was about .023 and now is about .026
the water change was roughly 1/7th
stock list
5 hermits (1 now exiled and one about to be buried)
1 dragonet
1 lettuce slug
1 either tux or pincushion urchin
3 anemones
1 mini maxi
1 atlantic
and one mystery bubble tip type thing
then a scallop yes i know theyre hard to keep we have many bottles growing phyto planton for him
then the pompom decorator and anemone crabs which yea idk

everything in the tank seems fine
dragonet is still being the dumb strange water hummingbird creature
hermits still knocking everything over
scallop still doing his best faulty wire impersionation
urchin still stealing my macroalgae for his fashion show
lettuce slug still making me question what planet it came from
and the corals open and waving around and just generally being a sentient rock

then again the things that died didnt seem that unhappy before

so is it just an unfortunate accident with the water change or could it be something possibly the just affect crustaceans
i really dont want my hermit gerald to die hes one of my favourite pets

im yea just everything about this situation
like one day i have to bury one of my favourite tarantulas and exile a crab which i still feel bad about
the next day i have to bury another two crabs
then the day after i find another crab dead and then have a site glitch out and waste over an hour of writing
so yea sorry if this seems breif or annoyed because honestly kinda am
honestly the words id use to describe this week would definitely get the post removed

 

[Sk8r]

I'm not sure what happened, but I strongly suspect the salinity change. Crabs are usually very tough. But shelled critters don't take salinity changes well, because the area they have to 'sweat' is restricted by the shell and their kidneys (or what passes for kidneys in a crab) can't take it and they die. Also beware of crabs with claws of large size---they aren't aggressive per se, but they will EAT anything including their tankmates. I like the critters, too, but no big claws are allowed in my tank, and unfortunately fish stores will occasionally sell them. To guard against salinity swings, get an electric hydrometer (I use one from Farmer's Supply, Amazon, cheap) and test the tank, wipe it down, then test the water you propose to use. The rule is no more than .002 change in salinity per 15 minutes, but that's for FISH. For crabs, I'd make that per 30 minutes or maybe even an hour. Sorry you lost the fellow. And as for any problem crab, in future, catch him and trade him or donate him to a fish store for somebody who has a tank that can handle him. Be VERY aware of the species of any crab you buy (well, except for micro-hermit you can hardly see let alone analyze)---but if it's a regular crab, know what you've got, because some don't like each other. And some get to be problems. A green mithrax took a nip out of my mandarin in a former tank---he went back to the store; and I also parted company with a sally lightfoot when I realized she grows to the size of a dinner plate. For a fun critter, I recommend the Scarlet Hermits, little fellows, but long-lived and very personable.