My experience with this problem is controversial! Randy has seen many of my threads and probably roles his eyes at me by now. I had a similar experience as you! And I'm 2 years into the nitrate journey (on a 4 year going on 5 year old tank) and my last test for nitrates was 7.5 on a high range test. My journey is controversial because I threw everything but a nuclear bomb (start over) at my 340 gallon display with 75 gallon custom sump.
First filtration I run:
1. Skimmer rated for moderate load on a 500 gallon system)
2. Algae turf Scrubber from Turbo Aquatics. Growns nearly basket ball sized balls of hair algae within 6-8 days.
3. I run a red sea reef mat 1200
I have 21-22 fish. They are huge though. 1 of my tangs is nearing 10-12 inches in diameter. My Male Creole Anthis is close to 10-13" in length (may be more, not sure, he's just huge). The other fish are fat and large. I feed 13 cubes of frozen food per day, plus a whole 8x10 sheet of nori per day.
My nitrates would go from 20 to 30, to 40, to 60. I kept upping my vodka dosing from 25 ml per day to 30, to 35, to 40 (upped 5 -10 ml per day on a weekly basis) allowing for a week for the change to take effect. I was up over 60 ml per day of vodka. going through 2 1.5 liter bottles a month. (I may have hand dosed more some days. . . )
I knew it was working because I had brown sludge (bacterial slime) covering my gyres, I had it floating in my sump, it would cover all the teeth in my overflows, raising my tank water volume because the teeth would get plugged. I had bacterial slime everywhere. My PH was falling as I increased my vodka dosing. I had gone from a consistent ph of 8 - 8.1 to 7.5 up to 7.6 during the day.
My tank was suffering.
The controversial part of this is because I blindly launched more equipment into the battle while vodka dosing: Things I tried while vodka dosing:
1. I tried Microbacter clean
2. I tried microbacter 7
3. I added a $300 pellet reactor (supposedly all I need to reduce nitrates).
4. I converted a cannister filter I had to a denitrator by filling it with enough matrix rock to treat twice my tank size.
5. I tried massive water changes (50% 2-3 times a month).
My nitrates dropped between 40 and 50 and hovered there. I tried backing off the vodka dosing and found that the nitrates stayed in the 40-50 range. Everything I tried was over the course of the last 2 years. Allowing for the pellet reactor to start working for 3-4 months before I tried something else. After 4 months Nitrates didn't change, so I added in the matrix rock. dosed vodka and waited another 4-6 months for everything to start working.
Finally, I gave up and was tired of the brown sludge covering everything, it was disgusting. My display was always so cloudy and fully of circulating debris that I just got tired of it all. I stopped vodka dosing all together. I did a 50 % water change to try to clear out the debris and sludge. Cleaned the glass really well, and tested nitrates weekly.
Each week I noticed my nitrates start dropping. (They increased significantly a week post water change from 25 right after the water change to 50 after 7-10 days). Then, weekly thereafter, they started dropping from 50 to 39. 39 to 35, 35 to 31. After a month, no water changes, and nitrates dropped in nearly half. Another month went by and they dropped to the 20s. I kept feeding, and doing no water changes. Algae was still growing massive balls on the scrubber. I'm 6 months in with no water changes since stopping vodka dosing and my nitrates have dropped to 7.5. Which on high range test, probably doesn't mean much, but, I know they're most likely between 0 and 15. Well within acceptable ranges.
My PH is now rising to a low of 7.8 at night and 8.03 during the day. (Not as high as i'd like but much improved.). Water is not nearly as cloudy as it used to be, and I no longer have brown sludge growing everywhere.
My suspicion is that at some point the pellet reactor and matrix rock kicked in with bacteria. But, Bacteria in the display from vodka dosing were outcompeting what was in the other reactors. I don't know if it was bacterial die off in the reactors that kept nitrates increasing (becoming nitrate factories instead of denitrators). Or what. But, once I stopped vodka dosing, the other denitrators kicked in and rapidly resolved my nitrate issues.
The matrix rock had an article from Sea Chem that they work best if nitrates start out below 20. I don't know why this is, but, maybe it was my 50% water changes that got nitrates low enough for the rock to populate with bacteria. I don't understand the science...
That said I'm at no water changes and nitrates down to 7 from well over 60. I'll take the win!
I'd recommend a matrix rock reactor or pellet reactor. However, my experience is it took them 6 months to a year to become effective for my feeding protocols. Could I have done something to speed that up? I don't know. Seachem and the Pellet reactor company both said they should become fully active within 8 to 12 weeks. That was not my experience.
I hope this helps! I now have issues with phosphate... So, one problem mutated into another.
