Ok....I need help!
About a month ago I got three beautiful C. davidsandsi. All of them were doing great until about a week and a half ago.
One evening, while I was feeding my fish, I noticed 'Y' (I name my fish

The next day he got worse, then he got better, and now he has gotten worse again. Y is eating, but his fins look slightly eroded (but they aren't getting worse) and he is less active and swimming always in the middle section of the tank, which is not normal. What could be a matter with Y?
Now, I have another sick Cory, another C. davidsandsi named Z. Z was the biggest and healtiest of them all, but suddenly two days ago she stopper eating and always stays in this one place! Noting looks weird on the outside, but I'm worried what could be happening in the inside. What could be a matter with Z?
Luckily, my third davidsandsi X is fine.
Heres some info about my tank:
-Temp is between 25-26C.
-pH is about 7.
-Nitrite, Nitrate, and Ammonia are not problem.
-I change 35% the water about every week and a half (plus I do a small vacume job), and every three weeks to a month I change 45-50% of the water and vacume the whole tank (I did that yesterday).
-Its a 54 gallon tank.
-The substrate is gravel, but smooth gravel with a little sand in it.
-Aquaclear 70 is the filter I use.
-The tank is filled with seven pieces of large driftwood and five big rocks. Also some plants (live abd fake).
-The tank is over two years old (or three, I forget lol).
Please help me and my Cories!
CL1