black spots on cleaner shrimp

[g. mcclean]

Our cleaner shrimp which has been in the aquarium for about 2 months has developed black spots on its body. Is this a sign that it is beginning to molt? Could it be a sign that water parameters are off? I did the full range of tests this AM and all seem fine with the exception of nitrate which is about 25 ppm. In addition to an Aqua Remora skimmer we run a Fluval cannister filter. We also have approx 55 lbs of live rock in a 46 gallon set-up. I thought I had read that the media in a cannister could actually harbor nitrate. Is this so? We run the Fluval as a "crutch" in case we do not keep everything in tip-top shape. The Fluval contains spong, carbon and bio media. I am also working on the calcium which is up to 360 and the alkalinity which is 3.5. Thanks for any help.

[kreblak]

Where are the block spots, and what do they look like?

[mogurnda]

Mine occasionally develop black spots before a molt. Never seems to be a problem. Once established, they are rather insensitive to nitrate, so I wouldn't worry about it in the short term. If well-fed, you should get a molt every 2-3 weeks.
The fluval will probably be OK for carbon and mechanical filtration, although probably not necessary. The biomedia, and the sponge if it's not cleaned regularly, will collect debris that will break down to nitrate, and this may be part of your problem. My choice would be to remove it entirely (my preference) or just keep it there with sponge and carbon for added circulation. You will need a lot more circulation than just the remora.
As far as the tank's general happiness, what else is in there, and how much circulation have you got? The Ca is OK, as long as you aren't trying to keep corals happy, but >400 is better. Alkalinity sounds good, though.

[g. mcclean]

In reply to your replies: What do the spots look like? They are tiny black dots primarily on the first body segment. The shrimp seems happy and eats what the fish eat. He also seems to have a "relationship" with the flame angel. What else in the tank? We have a flame angel, a red spotted sandhopper, a royal gramma, and 3 green chromis. We also have many hermits, a green emerald crab, and three kinds of snails. In addition we have 2 red gorgonia and 2 hammer corals that seem to be "happy". As for circulation, in addition to the skimmer and filter, we have an AquaClear 402 ( 270g/h) and a ZooMed power sweep 228 which does 270g/h. Thanks for your input.