Mysterious Corydora Death

Question
Dear Michael, I hope you can help,

Just to get the facts of my tank out of the way first. I have a 5 gallon tank with, now, 2 corydoras, one emerald (male), and one spotted (female), that has been set up for one year, I have an external box filter for 10+ gallon tanks and a heater, the temp is usually around 78 degrees, I unfortunately do not have current water readings, but I do do change approximately 20% of the water weekly.



I had up until a couple days ago another, albino (male), cory but he mysteriously died without any signs of illness, wear, or tear the previous day. I had had the same three cories since I set up the tank a year ago. Since his death, I've noticed that the other two cories are looking a bit lackluster and pale, and not quite as active as usual, and they also seem to be more paranoid than usual, whenever I get close to the tank, they start to get very nervous and go to hide. I realize that I need to do a water test asap, but I'm just wondering if anything other than just the most recent water change could have killed my one cory, and made my other two act so strangely. Thanks so much for taking the time!



Meredith



P.S. Also I thought it might be pertinent that I mention that I use AquaSafe conditioner and Stress Zyme Biological Filtration Booster with each water change and once a week respectively, along with maybe a quarter of a tablespoon of aquarium salt, as instructed by my LFS.

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Answer
Meredith,
time to pick up a test kit so we can determine whats going on in your tank.
you want one with PH, ammonia, nitrite and nitrate.
it is impossible for me to say what happened untill at least we can rule out the water parameters.
let me know when you get the information and we will try to figure this out.