Mistakes made, but I thought I was finally doing everything right

I just got up to find another danio dead today -- again, he was swimming around happily yesterday and there's not a mark on him. Water is 0-0-5 this morning (and it's tank-cleaning day). However, the thermometer said 82 degrees. I have my Hydor 25W heater set for 79 and I've never seen it go that high. I pulled the heater out and it was almost too hot to touch at the bottom. Could that have done it? The other fish in the tank seem fine.
 
Fish die for many reasons...and sadly all the time. The stock available to us are often weak from conditions, overbreeding and prophylactic use of antibiotics. They also may have been older fish. Like the others said, your tank parameters look great. I'd drop the temp back down to 78 and keep doing what you're doing. Like RBishop said, that tank is a little small for darting-around-schooling fish. I'd add a few more pygmi cories.

Mark
 
I do love the pygmy corydoras. Is there anything special about their care that I should be thinking of?
 
Bad stock with danios. I hate those things, especially the zebra that don't ever leave anything else alone. I had giant danios once, and those were great fish, but most others just nip way too much for me. Pygmy corys are just like other corys; good water changes and nothing sharp in the tank to hurt their pretty little feelers.
 
I have soft sand on the bottom of my aquarium which should be good for the corys, right? However, I've lost 3 in exactly the same way. Within 24 hours of a partial water change (and usually much less) I find them dead with a red spot on one side of their bodies. Any thoughts?
 
Its hard to say. . . but it is a little strange that its right after a water change. If it is something going on with the water its strange that its just the danios. . .What are you using to fill the tank back up? Any chance there is some sort of chemical residue in it?
 
I use tap water. Our town has a state-of-the-art water filtration system and extremely high quality water. I use 1 drop of Prime per gallon (I put the drop in a bucket and then fill the bucket with water so that the Prime gets dispersed) and take the temp of the water with an instant read thermometer before adding to the bucket. The only thing I can think of is that when I pour the fresh water into the tank, even though I do it gently, it still disturbs the sand and particles of debris that were settled between the sand grains float up. I have the filter on low power because of the betta fish and maybe it's just not filtering enough filth out of the tank? Maybe that's enough to shock the system of the corys? I really enjoy watching them but I'm hesitating to get others.
 
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