I believe I went through some of the same issues in the past year w/significant losses eventually. Things were going downhill mysteriously with "good" test readings. Check your test kits btw. I had a bad Mg kit. I had my LFS run tests and found it was off. I was really 800 ppm on the low side. All other tests were fine. I took the feeding angle too, but I believe all of the excess feeding causes the rocks and sand bed to turn into a nitrate/phosphate sink making things worse. You weren't seeing anything measurable in the water column, but it is in these surfaces right where your corals are residing, and all that algae, cyano, etc starts growing. I believe the carbon and GFO may be stripping too much out of your water column and furthermore not helping the corals. I used GFO and removed it, haven't seen any negative results since. It wasn't until I started carbon dosing with RS nopo-x that I saw an almost immediate improvement. I took it slowly though, w /small doses. If I saw a little cyano I backed off and gave a day or 2 break. Algae started dying and then I did a few clean ups(much easier when dead or dying). Water got really clear and I could see my rocks, gravel and glass again. Skimmer would go nuts in the beginning of dosing which is supposed to happen. Now my doses are sparse, keeping an eye on things, dosing from time to time. I am hoping to achieve balance in my system again, not taking any drastic actions. Additionally, I noticed a top film in sections of my sump. I cleaned them out thoroughly and added airstones in 2 sections to breakup surface tension. I also was dogged with lowish pH (7.8-7.9), and also suspected too much CO2 in my tight house built in '03. Never had this problem with prior tanks in older homes. I run a Ca reactor which can make this worse. I ran an old Aqua C skimmer which was rated to handle about my amount of gallons ~130 g. It wasn't suited for pulling outside air like a venturi system would. After lots of debate I upgraded to a Skimz SM203 and ran outside air to the venturi. pH now holding at 8.0-8.1 all day and night. So, here's the thing on skimmers. I significantly upsized to the 203 which Skimz rates at 528 g whereas BRS rates it at 190 g heavy load. Your Oct 150 was rated by BRS heavy load 55 g. So it might have been fine but underrated for your application. I feel like the co2 / low pH thing was an over riding source of problems in the tank. Something that was new to me, making it more confounding. I was taking all kinds of other actions that made things worse. I now feel the tank is much more oxygenated, in the proper pH range and has re-acheived the balance it once had.
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