My catfish's tail is turning red - what is wrong?

[Shell]

Help! I am a new fish owner. I bought a tank ensemble about 7 months ago. It was already aged, with fish, water, plants, light, thermometer and filter all supplied. In the first few months I had nitrate problems, but that now seems to be under control.

The current set up is as follows:

Tank size: 30 x 30 x 35 cm
4 guppies
3 catfish (cory's I think - looks like a c. ellisae, but I really have no idea)
10 neon tetras
1 bristlenose (baby) - in tank for 2 months

I do a 30% water change weekly. I now keep a nitrasorb bag in the tank for safe measure as well.

Yesterday I noticed that one of the cory catfish's tail looked like it was bleeding. It didn't seem that bothered about it, but it doesn't look very nice. Today, I have noticed a small hole in the tail near the body, the same area where the red is. He is hiding under the log, and hasn't moved all evening.

My local fish shop does my water checks, so I will go and get a sample checked tomorrow. But in the meantime, any ideas what I should do, or what could have caused this?

Many thanks for your help

from an abosolute beginner!!

[Marc van Arc]

Looks like a bacterial infection. Can't you take it along to your LFS and let someone have a look? Also, if I'm correct, you're going to need treatment. These diseases may be very contagious. I wish I could be of more help.

[Durlänger]

Is the size of the tank realy in cm
If so you need a bigger one, because there are to many fish´s in it
Cory`s like tank with more then 60cm front