Dying Cory Fish

eaglesaquarium said:

What is the tap water reading when leaving it to sit out for 24 hours?

I don't really have any advice but I just went through the same thing with all of my cories... I lost all 4 of them to the same thing. One day they were fine, the next they were either dead or going limp. Water parameters were fine, no external damages.... I think what caused their death may have been the heater. Mines around 80-82F and I also have a heater that's non adjustable, so it's a one setting temperature. What's weird though is I had used an Ich treatment (Kordon Plus) and they started dying not shortly after that. My conclusion is either the higher temp. waters killed them or they had long term affects from the medicine. 
Wish I had some tips for ya but I understand what you just went through. It's extremely stressful to not know the true cause. Hope you have more luck next time

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Corydoras are notoriously sensitive to meds (as many scaleless fish are).  Generally the recommendation is to go with a half-dose for twice as long with corys in the tank for ich.
 
Also the species of corydoras matters in regards to the temps.  Sterbai can handle higher temps, but other species really can't.

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Yep, I did half doses and daily water changes but something was triggered and the death began. The only thing I would recommend is just being extremely cautious, because I was well aware of what to do yet they still died. I would never use Kordon Plus with corys, even though it was the most recommended product... I don't trust it anymore.

I took a 20 hour water pH reading. 
 
Tap water sitting out was at 7.0. 
 
Tap water with dechlorinator sitting out was at 7.0.
 
The tank is reading at least 7.6.
 
The last cory is still alive.  He barely moves.  I don't know if that is because he's sick or if he's lonely.  I also unplugged my heater, and the temp is reading about 78deg F.  If I changed my filter today, how long do you wait before doing a water change?  I don't want to stress the guy out too much.
 
Thanks!

"changed your filter"?
 
What else is in the tank, besides the fish?

 
This is a small shot of my entire tank.  It is the Aqueon Deluxe 10 gallon starter kit.  It has the Aqueon QuietFlow filter.  I changed the carbon filter media today.  They want you to change it every 4 weeks or so, but I decided to start using that reusable sponge media instead of the discardable carbon stuff.  I don't know if that makes sense...
 
The cory actually seems a little more lively since I did that. 

Yes, that makes sense...  There's no need for carbon in the tank on a regular basis.  It has its place, but primarily for removing meds.  Not for everyday 24/7/365 use.
 
 
Still not sure why the pH would be so much higher than the tap after sitting out...  I'll have to think on that a bit more.

Oh, and the other things in my tank include: basic aquarium gravel, 1 rock (aquarium grade), 2 small pieces of wood that are supposed to help slowly lower pH, 1 fern, a plant I can't remember the name of, and one very slow moving cory fish.  I saw a snail the other day (they probably came in on the plants).  They seem to show up sporadically, and I quickly remove them from the tank.  That makes me think there is stuff in the gravel that I don't know about.  I'm wondering if I need to take out all the gravel and replace it with new stuff.
 
Thanks!

I wanted to say I just did a 30% water change.  Right after the new water was added, I tested the pH.  It still reads around 7.8.  The new water I added was around 6.8-7.0.  Should I take out all the gravel in case there is something in there?

Don't remove the gravel, that could cause your tank's cycle to go wack since it has the good bacteria in it. It'll make your tank recycle which could cause more stress on the fish.

I'd definetly switch out your gravel....and the rock is "aquarium grade" meaning you bought it from a fish store? I'd put some of the gravel in a bucket with tap water and let it sit overnight. If it changes than remove all gravel. Same with your rocks. Be warned though, if you change your substrate than that can cause a mini cycle if you don't have enough bio media in your filter. Corys do way better on gravel and I would suggest switching anyways. Go buy a new heater, I use marinas and they work fine. Do 30-50% water changes daily and cut back on the lights. Don't treat with ich medicine or salt. If he does have white spots than your temp of 80 is fine, if he doesn't have white spots that's too hot for most corys. I have sterbais that I keep in 81, but I wouldn't keep them in anything less than a 20 gallon. If it is your gravel, and you don't have the option of removing it I would rehome the Cory asap.
Sand I meant not gravel..

Honestly, I'd not fear the mini-cycle at this point.  All that is left in the tank is the single cory, correct?
 
 
In many situations, the mini-cycle is possible, but the truth is that in this case -there's no real concern.
 
 
The bucket plan is a good option.
 
I'd start by testing tap water in the bucket day 1, with NOTHING in it, except dechlorinator.
Day 2 - test the bucket of dechlorinated water with just the aquarium rocks (easier to remove than the gravel).
Day 3 - test the bucket of dechlorinated water with just the gravel.
 
 
If either the gravel or the rocks cause the pH to rise, I'd remove them from the tank permanently.

How often can I do water changes for him?  I took out a few scoops of gravel, which kicked out dirt, so I vacuumed it out and did a mini water change.  Can I keep doing that every few hours to get the gravel out?  The rocks I have did not affect the pH.  I'm worried about stressing the little guy.  He looks healthy other than not moving much and not really eating.

They get a bit sullen when they are alone.  

Should I get him more friends?  Or is the tank in too much of a flux?  And after scooping out a bit of gravel then doing a 25% water change today, this cory is swimming vertically by the filter box.  I have noooooooo idea what that means.  Maybe he just wants to hide...
 
So far, the gravel I scooped out hasn't changed the pH in the bucket.  I feel like scooping out more gravel, but I don't want to disturb more dirt.

Sorry for the mass postings, but I keep coming up with more questions.  And I don't want to kill another fish...
 
Every time I do a water change, I am adding tap water with a pH  around 6.8 to the aquarium with a pH around 7.6.  So I'm afraid of pH shocking the cory with all the tap water.  But I tested the water RIGHT after adding the fresh stuff, and the pH was already at 7.6.  So maybe the tank is even higher than 7.6...  But I want to know if I should stop changing the water since the pH if so different.  Or do I need to do something to raise the tap pH?  Oh goodness...
 
He's still hanging out at the top like he needs air.

How much oxygen is in the tank (is there a bubbler or anything)?
If you want to keep the water pH around the same, perform daily waterchanges of the same percent each time, this should (hopefully) keep the pH around the same.