Found some popping back up on my back glass this weekend in the 20 long...hadn't noticed it until I went to clean the tank and some detritus stuck to it. This is the second time it's popped up...the first time I solved the problem my adding the mp10. This time I ended up doing a 3-day blackout...today is day 3. I checked last night and it seems to be working. I'm going to run just blues for a couple days until I can mod my filter and try to get a little fuge going to see if that helps. Although I'm sure there are multiple factors that can contribute to an outbreak, I'm pretty sure the cause in my case is the pH drop at night. If the fuge doesn't solve the problem, I may try to run my tank without the glass top...but that will definitely mean hooking up an ATO to my system because I'll end up getting a lot of evaporation and would rather not do large daily top offs in such a small volume of water.
I haven't changed anything in the tank recently that would've caused the outbreak, and have an extremely small bioload...the only difference between now and a month ago is that it was unseasonably warm at the start of December, and I had the windows in that room opened for most of the day.
Edit: I did find one other issue...my salinity had crept up to like 1.028 somehow...I must have mixed my salt incorrectly a couple weeks ago. That's what I get for doing a WC with a fever, lol. I'm not sure if a low DOC would've contributed as well, but it's a possibility for sure. I corrected that too, so hopefully it helps solve the problem. Checked my refractometer, and it's calibrated...ordered some new DI resin for my RO/DI as well...it's been about 5 months since replacing it; I may start replacing it every 3 months since cyano seems to pop up in my other tank on a 3 month schedule after changing my filters and resin.