Aggressive crabs?

[spider crab]

My six line wrasse died. I just bought him Sunday an he seemed heathly, he ate well immediately. This morning I found one of my emerald crabs eating him. I was wondering how predatory crabs can be. I have several emeralds, a decorator and a sally lightfoot in tank.
Is it possible that one of the crabs is catching my fish and killing them?I had a mandarin die a few weeks back as well.
My water is within the correct parameters, though my nitrite spiked a little earlier this week. I got a reading of .3mg/l, which is the next reading up from the lowest my kit will test. I did a water change and that seems to have corrected it. Could that small of an increase in N02 kill a fish in a few days? I'm assuming it spiked a little due to adding the 6 line wrasse.
Other inhabitants in the tank include a diamond goby, a fiji bar goby and a tomato clown. I also have a condylactus. I also have around 32lbs of live rock and a few mushrooms and star polyps.
All of the other inhabitants appear to be healthy.

 

[cap'n pete]

I can't say for certain that your crabs didn't kill your fish, however that's unlikely. Ever heard the old adage "Where's there's smoke, there's fire!"? Well... where there's nitrite, there was ammonia! If you had detectable nitrite, then you had an ammonia spike that you missed. This very well have stressed the fish enough to kill it. My suggestion is that you look into increasing your biological filtration (add more LR) and DON'T add any more fish.

 

[cap'n pete]

So your saying you have six fish and an anenome in a 29G with only 17 lbs of LR? WAY TOO MUCH BIOLOAD. For a 29G 3 to 4 small fish in a well stabilized system is pushing it.

 

[spider crab]

So its more likely I'm overloading my system than the crabs? I do have around 32 lbs of LR now, Guess I need to update my profile.
Do things like anemonies contribute much towards bio load. I guess I had it in my head that they didnt.
There 3 fish in the tank now: diamond goby, Fiji bar goby and a clown. I guess the gobies couldn't be considered small, although the diamond does contribute to water quality by sifting the sand on the bottom. or so I've been told.
Do the crabs contribute negatively in terms of bio-load? I was thinking they are serving as cleaning crew and therefore reducing ammonia and nitrite build-up.

 

[thepomonacow]

Everything that is living contributes positivly or negitivly to your bioload. The waty I look at it is this: If it poops, it increases your bioload. If it doesnt rocks, sand) it helps lower the bioload

 

[panhead]

have an emerald crab I straight up caught hiding
in a piece of coral trying to ambush my clown.
He just hid untill the clown got close and then
swung out and tried to catch the clown.
also lost a scooter blenny and a manderin goby
(long time inhabitants till crabs arrival) have
since gotten rid of crab replaced the scooter blenny and no new losses. wish I could find another healthy manderin.

 

[spider crab]

Although it seems clear that Ive overstocked, I still can't help suspecting my crabs to a certain extent. Perhaps a combination of weakend fish from overtstocking and aggresive crabs. I've seen my sally lightfoot a take jump at my fish before and sometimes the emerald's do the same.
I really like the crabs alot though. They are facinating to watch, if not annoying when they move and knock over my mushroom and polyp rocks.
Oh, well thanks for all the advice!