help!! Lost a cory =( nitrite

[squid]

Need help!!
I lost a Cory yesterday, I noticed in the morning that 1 of my cory was swimming around as if in pain
when i got home at night it was already dead, but my other cories are not affected .

tonight I tested the water and nitrite was .25 ppm.
ammonia 0, ph 7.5
the tank is a 75 gallon tank with about 20 pcs cories.

filtration is 4 foot overhead filter box with lots of ceramic rings and a sponge filter

any advice? what i did was dissolve 2 tablespoon salt and added to my cory tank.

any other thing i should do? where could the nitrite come from?
I just did a 20 5 water change nioght before and added some csm+b to my tank for my anubias plants.

what should i do next?

[MatsP]

Let's start from the back, and say that nitrite comes from the ammonia that the fish release as waste from their metabolism. Your filter, assuming it's working as it should, has nitrobacteria in it, that converts ammonia to nitrite and nitrite to nitrate. Nitrate is roughly 100x less toxic, so it's a good thing to have that.

Large water changes is the best way to get rid of nitrite.

--
Mats

[squid]

Let's start from the back, and say that nitrite comes from the ammonia that the fish release as waste from their metabolism. Your filter, assuming it's working as it should, has nitrobacteria in it, that converts ammonia to nitrite and nitrite to nitrate. Nitrate is roughly 100x less toxic, so it's a good thing to have that.

Large water changes is the best way to get rid of nitrite.

--
Mats

thanks mats, the tank has been up for a few monthsn now, i just remembered 2 days ago my wife fed my cories and she overfed them tetra bits was all over the floor, but i did not di a water change immediately , should have done 1, thanks. i'll continue to monitor and update.

[squid]

Need help!!
Last night I lost a sterbai Tested parameters and
ph was at 7, No nitrite and Ammonia,
did not test for nitrates yet.

and this morning, I lost a pc of rabauti, will make water change,
but not sure what cause the 2 to die? what i did was just feed bloodowms, and dropped a sera activant,
really puzzled. I checked barbels of the remaining cories and they seem to be fine?