Losing Peppered Cories - Not sure why

[abcdefghi]

Over the past few weeks I have begun losing peppered cory cats from my tank, all other fish are acting normally, but I am slowly losing cory cats. My tank has remained the same for at least 2 years, and has been set up for over 3 years. The only recent changes in the tank are the addition of some plants (which had snail hitch hikers), switching existing driftwood for new driftwood, and removing some old plants. The new plants have also been added to a second tank, which contains a juvenile angel fish and pleco fry and no fish are affected in that 2nd tank.

1. Water parameters
a) 78F
b) Can't remember exact, but it is high.
c) See above, I have hard well water.
d) See above.
e) 0, 0, 15
f) Between 30-40% weekly.

2. Tank set up
a) 55G
b) Turface.
c) Eheim 2217.
d) Driftwood, Live plants (Java Ferns, Amazon Swords).
e) Angel, Black Neons, Neons, BN Pleco, Oto's
f) 3+ years

3. Symptoms / Problem description
Several weeks ago I found one of my smaller peppered cory cats dead in the tank, I put this down to a one off. Then, a few weeks ago I noticed one of my larger and older cory cats acting weird, laying curled up somewhat on his side at the bottom of the tank. When he swims off, his swimming appears fine. Since this fish is at least 4 years old I put it down to old age. However, I have now noticed one of the smaller cories beginning to exhibit the same symptoms. Laying on its side in a curled position, swimming normally, color appears fine, but I also noticed that they appear to have sunken in stomachs. All the other fish in the tank are fine, eating, swimming, good colors etc.

4. Action taken (if any)
At this point nothing other than water changes and monitoring the fish.

5. Medications used (if any)
Nothing. Not sure what to medicate for.

I hope someone can help, the cories are some of the fish I have had the longest and have survived through various issues including multi day power outages in freezing temperatures.

Thanks.

[N0body Of The Goat]

The stat that jumps out for me is the temperature, 78F (just under 26C) as a regular setting is rather high for Peppered and many other cory catfish. They should do much better at 72F (22C), perhaps with a small "hot season" where the temp approaches 26C. My guess is that these fish have had their metabolic rate highered and consequently their lives shortened.