Cory Cat not eating

[klj_current]

I have a 55 gal that has been established for about 6 months. The only fish that have been living in it is a school of 5 emerald cory cats and one chinese algae eater.

About 3 weeks ago, I added several schools of rainbows (a total of 14 new fish). A week after that, one cory suddenly died. I did a parameter check and all levels are fine:

pH: 6.8
ammonia: 0 ppm
nitrite: 0 ppm
nitrate: 0 ppm

All levels are still fine. Since then, I've lost another cory and another one is acting very unusual (unusual lethargy, loss of appetite, and is just not grazing and exploring like the other two.)

Can anybody help me?

 

[JustOneMore20]

How often do you do water changes? Do you have plants? You don't have any nitrates and that worries me. But, if you do large frequent water changes or have live plants, that would explain it.

It looks like you added alot of fish at one time so I'm wondering if you had an ammonia spike then, leading to the Cory deaths.

As far as feeding them, what have you tried to feed them? As you know, Cories hang out on the bottom so they need something that sinks. I feed mine sinking pellets and algae wafers. You may need to feed the Rainbows first and then put some sinking pellets in for the Cories. I think Rainbows are pretty greedy eaters usually, so make sure some food gets to the Cories. I have some greedy tetras that I have to feed before putting in sinking food, or they'll chase it to the bottom.

Also, how old is your test kit?

It's kind of strange for Cories to act like that without a noticeable reason (like water parameter issues). But, it does happen.

 

[klj_current]

I do a 10%pwc weekly. My test kit is is the API master freshwater test kit and is about 6 months old. I have been checking my parameters weekly with the water changes and there have been no spikes. There are no live plants in the tank.

The rainbows are indeed greedy eaters. I feed them flakes and then drop some frozen bloodworms in the tank in hopes that the corys can get to them. I will definitly buy sinking pellets/algae wafers tomorrow.

But if the corys were starving to death, wouldn't they get into a frenzy whenever I dropped food in the tank? Or are they so weak from not eating that they just don't have the energy?