I kept a saltwater tank as part of a year-long science project in high school with a friend of mine. All our levels stabilized quickly, but then the nitrates skyrocketed. They were so high that the tests couldn't read them, way past the level of being considered dangerous. We did 10% daily water changes, and nothing. Then daily 20%...and nothing. We took more than half the water out of the tank and replaced it with new water, and it hardly brought them down at all. (Yes, I know now that it's veryy bad to do that--but we didn't lose ANY fish or inverts!)
What finally brought them down was AmQuel+. There was a huge difference after the first dose, and I believe a week later we did a 20% water change and added a second dose of AmQuel+, and then it broughts the nitrates down to zero. It didn't affect any of our other readings in any way, and we never had a nitrate problem again.
We also used saltwater Biozyme when we did the HUGE water change, so that could have played a part as well. But I still use AmQuel+ in my tank at home whenever I lose a fish/coral/invert, and I've never had ammonia or nitrites higher than 0, and I've never had nitrates higher than between 0 and whatever the next reading is.
I had a pH problem at one point, I don't think it was related to the AmQuel+, but I used Searchem pH Buffer 8.3 as recommended by my LFS, and it worked perfectly.
IMO, I love AmQuel+. I work at a pet store and I recommend it to all my customers who have problems with ammonia, nitrite, or nitrate, and everyone's told me it's worked for them. I haven't heard of any related fatalities, either.
This may help you as well, what I had in the tanks at the time of treatments.
I had in the tank at school (if I can remember):
1 largeeee 3-striped damsel
1 large blue damsel
1 smallish false percula
3 orange linkias
3 bigger hermit crabs
6 really little hermit crabs
And in my home tank right now:
1 medium-sized blue velvet damsel
1 medium-sized palse percula
1 small/medium yellow tang
1 smallish coral beauty
2 purple tree leathers
1 small colony of maroon mushrooms
1 small colony of green/blue mushrooms
1 pink-tipped haitian anemone
1 bubble-tip anemone
1 cleaner shrimp
1 sea slug (it's light blue and yellow, very pretty, dunno what kind)
1 toadstool leather
2 colonies of green star polyps
1 colony of reddish star polyps
and a whole crapload of turbo snails, little blue-legged hermits, little scarlet hermits, and live rock.
I agree with trying to find the source of the problem of course, but I also think you shouldn't waste time trying to figure it out when you can act now for at least a temporary solution to your problem. After all, you don't want to lose any livestock. Good luck and I'll be happy to help in any way I can. :joy: