Planted Tank Guru
Join Date: Nov 2009
Location: Missouri united states
A few thing's I might look at.
Majority of cory's will do better longer in cool ,oxygen rich water with temp's between 73 degree's F and 76 degree's F.
could be difference between fishes that live but a few week's/month's, and those that live year's.
Cooler temp's hold more oxygen than warmer water ,and cooler water can help slow possible bacterial pathogen's.
Is wise to QT fishes, and you have saved other fishes from possible spread of pathogen's unknown.
With bare bottom QT, one can observe fish poop to see if it is solid looking and of similar color of food's offered as opposed to clear/white stringy looking which could indicate intestinal parasites.
Mixing food's with anti-parasite med's would be helpful if it is the latter of the two.
Will assume, (alway's a bad thing) that there is some type of filtration on the QT(sponge filter's work well) in which case I would not get too aggressive with cleaning the material for without substrate,,the lion's share of the bacteria will be found on the filter material and maybe some on the glass.
If no filter is present,or filter is not mature,,then daily small water change with conditioner such as PRIME that addresses not only chlorine/chloramines,but also ammonia would be my choice.
Would feed fish sparingly with fresh food's, not expired food's perhap's once a day, or every other day tiny feeding's no more than you actually observe them eat in a few minutes rather than dumping too much food ,too often in small tank.
New life Spectrum pellet's for small fish,hikari sinking wafer's(these are small),shrimp pellet's,and quality flake food's work well for these fishes and are but a few that I use with good result's.
Would see that no one else is also offering food's.
Would not offer frozen bloodworm's, but that bit of advice is just my personal opinion based on troubles with numerous species that seemed to go away with the elimination of these sewer larvae with other fishes as well as the cory's.(opinion's/expierience's vary).
Would not share net's ,bucket's,vaccum's, used in quarantine with other tank's without bleach /water soak to help prevent possible spread of pathogen's unknown.
Hope some of this help's.