Help! Sudden Decline in Cory Health!
First image from ~1 hour ago. Sick cory on left, healthy cory on right.
Second image from a few minutes ago.
29 gal low tech planted tank, no ferts/CO2. 10 Agassiz's Corydoras, 11 Black Phantom Tetras, ~14 amanos. Tank regularly has the following parameters: 0 ppm ammonia and nitrite, 10-20 nitrate, 6.6-6.8 ph, 4° gH, 0-1° kH.
Left on trip two months ago, returned one month ago. Tank sitter fed according to my instructions using a measuring spoon to be sure. There was a brown algae bloom while I was gone that left several plants coated in grey, presumably dead algae, gH dropped from 4° to 3°, and all but 1 of my amanos vanished. Fish all seemed lively. Did a ~40% water change but did not have time to remove algal gunk. Found one cory dead two days later with no preceding symptoms, assumed either the gunk was toxic or wc volume was too high for the first in a month. Waited a few days, vacuumed up as much dead algae as possible, no further issues until yesterday.
A week ago, did another algae scrub and 33% wc. I also moved a piece of driftwood to improve flow, but vacuumed under it as it was moved to minimize effects of anything trapped underneath. Fish seemed fine until yesterday. I noticed one cory was pale and not moving much. Normally, they are pretty active, will flee when I get really close to the tank (except at feeding time), and sit with their snouts pointed a bit upward when at rest. The sick(?) cory had a more slumped posture, and didn't respond to me tapping on the glass in front of it. All other fish look/act normal. Tank tested same as normal, but slightly lower nitrate (5-10 ppm).
Today, I came home from work...didn't notice anything different. An hour later, I find sick cory on its side near the driftwood: very pale, dark-eyed, barely breathing, and under attack by the last amano. I net the cory into a breeder box so they can at least die without being eaten alive, and tip it upright — cory struggled weakly but couldn't right itself. ~30 minutes later, its eyes and body have regained a bit of colour, but it's still breathing very shallow.
Questions: What happened to this cory? What caused the other cory's death a month ago? Is there anything I can do to help this one recover or should I euthanize?
Update: cory has passed. Help/advice still appreciated.