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I have a mature 29 g tank running disease free for over a year. One day about 3 weeks ago I wanted to change a type of fish in that tank, so I sold a pair of egg laying angels and bought 6 small rainbowfish.
And few days later my problems began.
First noticed rainbows had long clear poo. I had them fed well so that they would be strong in the new tank, but there was a lot of long clear poo.

Fish was from reputable store (BigAl) I spent some time examining the fish, I did not notice any sign of parasites in their tanks so I treated for bacterial infection, and only the new rainbows.

The way I did it I soaked dried blood worms with a bit of EC (erythromecyne?) and gave that to rainbows only. Did that two times. Other fish did not get these worms, positive, they got regular blood worms.
Clear poo was gone in 2 days and rainbows were eating like there was no tomorrow.

A week later I noticed a trilineatus cory wobbly and very dark. Died 2 days later.
Now the other trilineatus wobbly swimming, dark gills and yellow belly, rugged tail. It stopped eating and it looked bad. Was very white on the body with contrast black stripes. I just euthanized it, there was no hope, was laying on the side.
Just noticed an Otocinclus very dark, not eating and barely swimming, wobbly. This one looks very bad too, I have to euthanize him or things can go out of control.
All rainbows look healthy.

I do not know what to do,
At present I am treating with medicated internal parasite food from Jungle, second day, treating all the fish in the tank.
Can you point me in the right direction?
thanks

One detail I forgot to include in my post above: euthanized cory trilineatus had rapid breathing, red gills and yellow sides (belly area). But it was more white then remaining two, so maybe yellow belly was just showing more.
I did not change cleaning habbits.
I did lower temp by 2 deg from 80 to 78 when angels were gone and rainbows were added.
Tank has always been heavily planted and for over a year trouble free.
Rainbows are the only change, but they appear very healthy.
At the time rainbows was added, some stress on fish was imposed, tank was relocated, so I almost emptied it and moved when it was light, same water was put in from clean containers where it was stored, no temp was changed, I added some new water with the old one to make sure temp is stable.

Long post, sorry.
Thanks in advance.

I'm wondering if during your relocation, some bad water came up from out of the gravel?

Hi Kana3,

Thanks for the reply, you were the only one, probably my post was too long to read through.
I vacuumed the sand before relocation by sucking the sand half way up the tube and pinching the tube to slow down the current, that takes bad water from the sand.

The reason for the problems were the internal parasites brought by the rainbows. I treated for that with medicated food made by Jungle. I have had no more problems since that last sick cory around mid November. Medicated food looks very effective and you do not need to add carbon to the filter to get rid of the medicine, just vacuum more often during treatment.