I found The smallest peppered cory early this afternoon dead in a clump of hornwort, I assumed it was from stress after moving them to the big tank and sadly scooped him out.
I did a 15% water change just to be sure as i was worried about any pollution from his little body.
Checked the levels and everything is as it should be, no changes in temp, ph or nitrates.
Before moving them to the big tank we drained about 50% of their tank water and dripped in water from the big tank over the course of about 6 hours to get them acclimated, the water should have been fairly similar anyway since we do regular water changes and its coming from the same source.
All seemed fine tonight until about 1 am when I happened to notice a flurry of activity out of the corner of my eye and saw one of the cories swimming like mad and all spazzy trying to get to the surface.
I noticed blood on him so we quickly got out the gravel vac and clean bucket and siphoned out some water to be able to see what was going on. This took maybe a minute and half total and it was already too late, he was still alive but barely.
After he passed on we were able to get a good look and it looks like something was ruptured inside his little body, there was a pea sized area on his side between his gills and side-fin of bright red blood, but under the skin.
It has already faded a lot so im sure the same thing happened to the other guy but was already very faded by the time we found him
Everyone else seems healthy, the remaining three cories are fine, but so were these until whatever is happening happened.
Any ideas?