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1. Water parameters
Temperature 24-26C
pH - 7.0-7.2
Gh - about 12
Kh - about 8
Ammonia etc - 0 (NO3 5-10 cos im adding it by myself, its plant tank)
Water Change every week, about 20% of tank.

2. Tank set up
a) Size. 240liters
b) Substrate. JBL Manado
c) Filtration. Tetra Tec 700
d) Furnishings. Just a plants
e) Other tank mates. Corydoras panda, pigmey and paleatus. Also Poecilia sphenops (ONLY AENEUS ARE ILL!)
f) How long has it been set-up? About 2 years


3. Symptoms / Problem description
Corydoras aeneus are staying at top of tank, and seems to be not able to swim down, it started long time ago but now they get so slim and weak... I made two films of they:
http://www.youtube.com/watch?v=_m6ABUN3Wog
http://www.youtube.com/watch?v=UVReU2fFgrc

4.Action taken (if any)
I took them into another tank, with SAME water, but without filtration etc. they were much better!
When I took them back to main tank, they started doing same.
Also I tried to get bigger water change, cut off co2, turn off light for 2 days etc. nothing helped

5. Medications used (if any)
One of my corydoras died some day, he was not doing like aeneuse's but I found he got a yellow "thing" at himself (other's have too, but I think its normal for Corydoras) I took a photo of it:
img404.imageshack.us/i/1002640h.jpg/
I though it was a Oodinozium disease so I used Tryplavine into my main tank. Not changed anything.

Did you tried too add some oxygen to the water?
I think there is too less oxygen in it.
Put a airpump on it and see if that will work out.

this distinctly looks like a lack of oxygen - i had a similar problem turning off my airstone.

Unfortunately my L128 blue phantom pleco died - get an airstone and pump and you should be sorted!

either a lack of oxygen, or something poisonous.

I dont really think so, this aquarium is flora one, it includes many plants which are boubbling everyday.
So air level is max which is possible, I took out styrofoam from bottom which was at tank for about 5 years now... We will see it will help.

not... they are doing the same

Assuming you're right @ oxygen, than there must be something toxic in the water

I do recal other fishes seem fine, but than it must be something the Corydoras can not have and the others can

The only solution would be to empty the tank and look everyting over. Alternatively you could remouve the Corydoras