After having used biopellets for about 7 months, I experienced the overstripping of nutrients phenomenon recently. Lost a colony (Hawkins Blue Echinata) and 6 to 10 smaller frags/mini-colonies (4 may recover). Most were SPS, though I also lost a large colony of cyphastrea Meteor Shower, and even a fungia plate lost a lot of color, as well as a medium rock covered with porites. At first I thought it was the oft-reported alkalinity sensitivity of sps tips (went white), but then it spread to the rest of the coral. When it started a couple months ago, I removed about 3/4ths of my BRS biopellets. Then, with continuing degradation of my corals, I took out all but about 50 grams of the pellets. A week or two later I removed all pellets. Still no improvement. Finally did a 1/2 water change, and changed the carbon while about doubling the amount I normally used. Also have been dosing amino acids - and I have a fairly heavy bioload so nutrients are making it into the system. The colors are just starting to come back, and the polyps are again fully extended on most.
I was thrilled with the early results using the pellets, algae disappeared, didn't have to clean the glass but once a week (and only then to keep the corraline algae from taking over). Almost no green algae whatsoever, except for the Halimeda which took over my refugium over caulerpa and chaeto (which stopped growing and became a detritus trap, so I tossed it). Everything looked sparkling clean - but then my corals started fading some, slow to no growth, then the white tips.
Probably should have reacted faster, but didn't want to shock the system by just removing the pellets all at once.
Some general observations:
At first, the pellets seemed to harbor a biofilm (at least when they would clump and I'd have to break them apart a slime layer seemed to dislodge). But after a few months, it seemed there was no biofilm/slime layer on the pellets. I think what was happening was the pellets continued to dissolve (biofilm or not), and add dissolved organic carbon to the system (as a breakdown product - whether it's the monomer or some other breakdown product from the pellets I don't know - but would like to - perhaps I'll let some pellets sit in some clean fresh saltwater and do a quick extraction and inject onto a GC-MS and see what shows up). Anyway, it may be that the residual dissolved carbon in the system serves as a carbon source for bacterial growth to continue stripping nutrients (N and P) out of the system. At least that's my hunch. The activated carbon I always run may have become prematurely loaded with whatever organic comes off of the pellets, and adding fresh activated carbon may have scavanged what was dissolved in the water, finally improving matters. Again, just a hunch.
The lack of green algae made the display tank look nice and crisp. Didn't have a cyano problem either. But apparently that's not a good thing - probably need to adjust the amount of pellets, or the flow (recirculating?) to achieve some steady state low nutrient condition. But in the long-run, that may be elusive, and now I'm a bit leery of biopellets.
The corals that normally like a bit of "dirty" water didn't seem phased. My mushroom problem is still a problem. The LPS and soft corals didn't miss a beat either (the fungia excepted). Zooanthids never looked better. So can't reconcile that with the nutrient over-stripping idea.
Perhaps I had too many pellets - I was using the full bag - 1 liter on a total system volume of about 220-240 gallons. But for now I'm going biopellet-free, but may go back to a very small amount, or may just go back to vinegar which seemed to be the most trouble-free carbon source for me so far (I had problems with vodka - dosing that is).
