Change in Tank Chemistry and Bubbles!

[terickuehn]

I have a 120gal tank that I have had running for years that has started going downhill (corals not doing well, slight algae). Tank - 2 to 3" sandbed, 1/3 to 1/2 of tank is live rock, lot of muchroom corals (doing well), bubble, cactus, gorgonian, hammer, lots of coralline algae, 1 tang, 2 urchins, 1 lemon peel, 1clown with 1 (300w) MH and 4 (96w) PC's. Drop tank with 1/3 bioballs, 1/3 of it I have my Redsea Berlin skimmer in and the other 1/3 have an Oceanic filter that is driven by a separate pump that goes to a chiller. Cal:400, Alk:9.6dKH, Phos:0.1ppm, PH:7.8, but Nitrates have jumped up to 25ppm and bubbles are forming on the live rock and notice a lot in the sand bed. Water changes are only holding Nitrate to 25ppm but cannot lower. Clean Oceanic once a month w/power sprayer along with water change of 10 to 15%. Only feed frozen foods or kelp every other day. Fish are fine but what could be going on? Any and all help would GREATLY appreciated.