Please Help Our Corydoras!

[Fishomat]

Firstly many thanks for reading this!

We have a small tank including 4 blood-fin tetras, 3 cherry barbs, 3 peppered corydoras and 3 albino corydoras.
Well we did... The blood-fins came first, then the peppered corydoras, then the cherries and the albinos arrived at together. Things were fine for a 2-3 weeks days, then one by one the albino corys died. The first one we found wedged under the aerator, when we removed him he had reddening under the gills. The next one was dead on its side with browny red marks around the gills and we think the third one was similar.
Now one of the peppered corys is gone too - found hanging limp in a plant - we got him out of the plant, he was still alive for a bit, then we later found him on his side with the red around the gills too.

All other fish, including the other two pepper corys seem fine.

There is melafix in the water at present.

Tank size: 35L
pH: 7
ammonia: 0
nitrite: 0
tank temp: 26

The tank has been up and running since June - we had a bit of a fiasco with a few neon and cardinal tetras to star with before we moved on to the others. We lost a blood fin under the filter a couple of months ago, but all been fine apart from that. The filter is running, we have a working aerator, some living plants. The tank has a fair bit of algae, which we clean once a week and we do a 10-20% water change with bottled water once a week.

There are one or two tank ornaments and the substrate is fine gravel. (2mm or so in diameter)

Any assistance would be much appreciated as we don't want to lose anymore fish!!

Thanks.

 

[WILDER]

Your tank was overstocked.

What test kit are you using?

Red patches or bleeding beneath the skin can be septicemia.
The brown patches were they fluffy like cotton wool.
Any signs of flicking and rubbing, darting, excess mucas, laboured breathing or gasping.

Whats the ph of the bottled water.


Acidosis



Symptoms:

The fish’s skin may show excessive mucus secretion, a milky appearance, or reddened inflammation. There may even be erosion of the gills. The tale-tale sign will be the fish gasping for air just under the surface, exhibiting sudden, rapid fin movements and darting backward in the water. It may also attempt to jump from the aquarium. Sometimes a brownish deposit will form on the fish, particularly on the gills. Death may occur either rapidly or slowly. Oddly enough, the fishes often die in a normal swimming position with their colors still not faded.



Cause:

Low pH levels.



Treatment:

Your fish must be placed in water with pH values within their appropriate range. You should test the water with a pH kit. If the pH levels are lower than required for the species, perform an immediate water change. If the pH of your water source is inappropriate, an alternative must be found. If you need to raise the pH of your water, you can use either Proper pH, coral, seashells, or limestone by adding them to the water before putting the water in your tank.