Need advice on a new issue developing with my SPS..

[trameaa]

So, I started to notice about three weeks ago that one of my SPS had essentially "lost" one of its tips. Basically the tip looked ragged, like it had been chewed off at the end, and had lost its color (just at the very tip). At the time, I noticed that one of my mithrax crabs was hanging out around it, cleaning things up, so I wrote it off as the coral being irritated with the crab picking at things and I moved on.

Well, here I am three weeks later, and now all the tips on the coral look like this. No bleaching of the branches, and the base is encrusting, but the tips look awful. Today was my tank maintenance day, so I did about a 15% water change, siphoned out detritus, and just generally looked things over.
Much to my surprise, I noticed that two more of my SPS are starting to do this.

Just to double-check things, I looked over all my parameters -

Temp ~78.5 (Medusa temp controller and Reefkeeper probes)
Salinity 35 (checked with refractometer)
pH 8.15 (lights out - Milwaukee pH and Reefkeeper probes)
Alk ~10 dkh (Salifert)
Calcium ~430 (Salifert)
Phosphate 0ppb (Hanna ULR)
Magnesium ~1400 (Salifert)

Just so you have as much information as possible, my setup consists of -
75 gallon Oceanic
~100lbs live rock
2 x MP10w ES
1 x MP40w ES
30 gallon sump
MRC MR2 Skimmer with pressure rated pump
MR1 Media Reactor (running BRS GFO)
2 x 250w Radium in lumenbright reflectors
2 x Galaxy select-a-watt on HQI setting
Programmable dosing pump supplying 3ml every 3 hrs of B-Ionic Alk
Calcium is currently being dosed manually
Lights are normally on for 6hrs per day, and I have four small fish that are fed about 1x day with Rod's food

There is no noticeable algae in the tank - the hermits, snals, and mithrax do a great job of eliminating any plant matter pretty rapidly.
The thing that jumps out at me is the 0ppb Phosphate reading, and the _relatively_ high Alk reading ~10. This was after a 15% water change, so the alk was probably higher than that before.

Anyone have suggestions/thoughts? I want to try to get a handle on this before any losses.

Thanks to anyone that has ideas to share!

 

[Logzor]

Try supplementing your tank with Nori at night time. It could very well be the crabs eating the nice juicy polyps at the tips (fresh and new - easier to eat).

 

[trameaa]

Wow... really... I guess maybe I've been naive about the crabs... they are a bit menacing at times. Heck, they aren't even scared of me when I stick my arm in the tank to clean. They rear up with their claws in the air when I put the algae scraper in, but I've never seen them act aggressively toward the other inhabitants - other than an acute awareness, like they're waiting for a weakened animal to have the misfortune of crossing their path :worried:
Hmmm... I hope I don't have to eradicate these guys. I picked them up about 3-4 months ago because I had some bubble algae. They made fast work of it, which I was thrilled about.
Well, I moved a couple things around today.. two of the pieces that I noticed issues with I've relocated to an area with better flow and a touch more light.
Guess I better keep an eye on things a bit more closely for a while.
The nori idea is good, thanks for the tip. They do seem to prefer algae when available, but I get the impression they'd eat just about anything available.

 

[reefsurfing]

Maybe, your adding more alk than calcium! This could cause imbalances and it could be causing tip burn.

 

[RRaider]

I think you are looking in the right direction. Lower your alk

 

[Acronic]

before you even mentioned mithrax crab I was thinking mithrax crab! What you described is exactly how I would describe the damage an emerald crab will do to an sps. Check the tank after lights out, thats when the crab makes its move on the coral.

Here is a pick of my colony that was being prayed upon by a rather large mithrax crab. I removed the crab and within days the coral had healed up.

 

[Dholmblad]

I have found hermitcrabs eating some of my sps every once and awhile. I will just thrown those into the overflow box.

 

[reefsurfing]

I would try to eliminate one variable at a time to narrow down the issue. First, remove the crab and see if the issue stops. If not then you'll want to try adjusting your alkalinity until the burn goes away. Make the adjustments slowly since your sps could be in a weakened state.

 

[trameaa]

Acronic - that picture looks _exactly_ like what I'm seeing! Photo looks great by the way, aside from the obvious. Irritated me big time. Beautiful coral with the tips bitten off like a piece of beef jerky.
The funny thing is, I mentioned that I had seen one of the crabs around this particular coral, and it was indeed at night just as you mentioned.
These crabs are different than the mithrax I've gotten in the past - they're red instead of green, and fairly large. They ate the heck out of some bubble algae though (which I've never been able to get the green ones to do).
If I catch one of these freaks pinching off the ends of my SPS it will be war :uzi: