Well I think I've stopped the carnage. I was not able to do a 100% water change, but with Ronau's tub and the storage bins I had I was able to do a 60% water change. I was hoping to let things just settle with no intervention for at least a week, but it turns out that even in 375 gallons of water my surviving SPS is still consuming enough alkalinity that 48 hours without dosing would have been as fatal as whatever was just going on.
I set up the doser again and have spent the past 4 days trying to get it dialled in. I'm basically back to square one on dosing solution rates, so I started with less than half the amount I used to dose and am slowly increasing it every day based on the results from my tests. Today I had to manually dose some of the alk solution as my dKH has fallen in to the 5's and that scares me. I just need to find the new sweet spot to keep it in the neighbourhood of 7.
I have however stopped losing entire colonies, and the only new tissue loss I've seen since the 60% water change has been on patches of coral that looked like it was already too far gone to save. I have a couple acros that have started to form plate edges along the margins of where the tissue died, and on a few the dead patches are perceptibly starting to shrink. I'm still not sure if a couple of my largest colonies are going to pull through, as I think "too far gone" happens long before the tissue actually pulls away, so we'll see over the coming months whether they get normal looking texture and colour back and start growing again, or RTN in the middle of the night.
I've done HEAPS of online research, and I'm almost 100% positive now that this was the result of a biopellet overdose. I've seen similar reports from people who've OD'd their tanks on carbon (both liquid and solid), and Randy Holmes-Farley on RC found that his tank has an upper maximum dosing limit of vinegar, above which his corals start to suffer. When I fixed my biopellet reactor, I should have treated it like I was setting up a new reactor on a new system for the first time, since my bacterial population was completely wiped out when I took it apart and cleaned it, and my modification allowed for at least 10X the flow through rate. Instead, I put the same amount of pellets I'd built up to over 2 years back in the reactor (about 3L), and left the new 1" effluent gate valve open at 100%. In retrospect I'm not sure why I was so baffled as to what was happening, I basically followed a step-by-step "how to crash your tank" recipe.
With no pellet reactor nitrates have been rising fast, however. At this rate, and the rate at which I do water changes, and the amount of water that I change each time, it looks like my tank would stabilize between 10 and 15 ppm nitrate. That is about 10 to 15 times higher than it's ever been in this tank's history. I'm worried about compounding the stress of the past month and a half by allowing nutrients to sky-rocket, and I'm not thrilled with the idea of daily water changes in perpetuity, so today I put the pellet reactor back online, but with exactly 10% the recommended volume of pellets for my system. I have a gigantic pellet reactor so it looks kind of silly, but I'm going to do it right this time. I'll track the effect this has on nitrates over the next 6 weeks, and I'll only add more pellets if the nitrates don't perceptibly fall.
It's going to take a while to get all my parameters to stabilize, and until they do I'm not really going to worry about what's going on inside the tank, as I don't really expect my corals to get back to their former glory until all the major ions and the nutrient profile remains constant for a period of months. Once the chemistry has been stable for at least a month I'll start to worry about the visual effect this has had - specifically the explosion of cyano that now blankets half the tank. For now it gets a pass. For some reason my sand is cleaner than it's ever been though, so from far away it doesn't look that bad.
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