Nitrates and Overloading

Question
QUESTION: I've got a 120-gallon SW tank with the following inhabitants:  2 yellowtail blue damsels, 2 tank-raised Oscellaris clowns, 2 blue-green chromis, and 1 royal gramma.  I've also got an indeterminate number of hermit crabs, perhaps 15 small (1/2") hermits.  Also, I've got 1 chocolate chip star, one green brittle star and two emerald Mithrax crabs.  The aquarium has existed only since about mid-September, and I've had some critter deaths; mainly snails, but one brittle star and one chocolate chip.  The Mithrax crabs have been either hiding or deceased for weeks.  My chemical levels are:  Ammonia = 0, Nitrites = 0, Nitrates = 20 ppm (mg/L), pH = 8.3, specific gravity ~ 1.025, temperature 78 - 80 deg F.  I've got an Eheim wet/dry cannister filter, Model #2229, a Top Fin (I think) 60 power filter, a flomax 1200 (I think) filter/pump, and a SeaClone 150 protein skimmer.  I do 20% water changes every two weeks.  

Please let me know if you need any more info to answer this question:  Is it advisable to add any more fish in the near future?

Thanks,

Jim G

ANSWER: Hi Jim. I see no reason why you can not take the plunge and buy yourself another fish or two. You have a very large tank and all your levels seem to be in check for any fish. Your tank is going on four months old and at this point should be well past cycling and should be at a stable point by now. Do you have a quarantine tank? This is a very important piece of equipment and I can not stress enough the problems and money that will be saved simply by quarantining your fish for a few weeks before adding them to your main tank. At any rate, go on and add another fish!

---------- FOLLOW-UP ----------

QUESTION: Hi Jennifer,

Thanks for your quick and informative reply.  I've got a sort of follow-up if you don't mind.  I was planning on picking up (this weekend) a yellow tang and one or two pyjama cardinals.  Is two cardinals pushing the limit?  Also, will the tang eat the red slime (blue-green algae) we have?  It's not overwhelming, but is of some concern.

Thanks again,

Jim

Answer
Hello again. A tang would be a wonderful addition to your tank! The cardinals will also be okay just be sure the damsels do not decide to start picking on them. Sometimes damsels will pick on any new fish added that are smaller or even slightly larger than themselves. Although cardinals can be a bit feisty and handle themselves very well, as a new addition it will take them a little while to learn to stand their own ground. Cardinals are also more of a nocturnal species of fish and will often hide during the hours of light. These fish prefer a subdued atmosphere and it may take several weeks for them to acclimate to more intense lighting in an aquarium. Having an area in your tank that is slightly darker will keep them out in the open more and give them a space where they can feel more comfortable. Unfortunately the tang will not eat red slime algae. They have a preference for green micro and macro algae and there should be plenty of these algae provided in order to keep the tang healthy. If you do not have a good supply of green algae in your tank, be sure to purchase some seaweed paper and a veggie clip along with your tang. Without greens in a tang's diet they can easily succumb to infections due to poor nutrition. Keep in mind that tangs are one of those fish that are very prone to ich infections and be very choosy when picking one up. Ask to see the fish eat before you buy and never buy a tang that has a pinched in appearance to its belly. Good luck with your new fish purchase!