Crustaceans dieing - HELP!

[pulik]

My crustaceans - 1 cleaner shrimp, 1 peppermint shrimp, 4-5 hermit crabs, and as of today, 2 hitch hiker crabs in my live rock are methodically kicking the bucket. Any ideas of what might be amiss?

Tank specs are: 20 gallon long (30"x12"x14"), new set-up (~3.5 weeks old), approx. 30 lbs of live rock.

Equipment: AC 70, Rio Nano skimmer, Seio 620 powerhead, 30" 36 watt coralife double linear T5 (10,000k & actinic), and 24" 48 watt Nova Extreme T5 high output (10,000k & actinic) lights.

Fish: 2 percula clowns, 1 purple firefish, 1 royal gramma, 1 catalina goby & 1 mandarin dragonet (eats frozen).

Other: 6 snails, 1 torch coral, 1 waving hand xenia, 1 fox coral, chunk of star polyp, medium sized rock covered with green hairy mushrooms, other mushrooms, 1 AWOL (though alive) anemone & 2 feather duster worms.

Chemistry: NH3/NH4 - 0, NO2 - 0, NO3 - 15-20, pH - 8.4/8.5, PO4 - 0, temp - 77.2 F.

Have been doing 10% water changes about twice a week, trying to bring down the nitrates. Also, adding ~15 drops of Aragamilk (calcium & buffer) on a daily basis, along with 2 ml. Reef Solution (minerals, trace elements & vitamins).

Everything is thriving - except the crustaceans; mushrooms are growing and spreading, xenia has grown considerably, torch coral & fox coral look great, star polyps are spreading, fish are active, eating well and have excellent colour.

Sorry about the long post, but hopefully everything necessary is listed.
...Any ideas???