Maybe you could give us some further details on your tank since you now have all our attention?
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As a total newbie I lost 2 guppies when I put them in a 5 gallon tank with my betta, per the lfs guy saying it would fine.
So, I got a 10 gallon, had NO idea about cycling a tank and lost probably 3 other guppies and a panda in the process of doing an fish in cycle that I didn't even know I was doing.
I lost one peppered cory mysteriously but I think he wasn't the healthiest fish because he had a crooked tail (which I found endearing).
So, I ended up with a 10 gallon with 4 guppies, an albino cory, a bronze cory, a panda cory and an otto. Once it was cycled and for the last couple months, the tank was doing pretty good. Everyone seemed ok and water parameters were under control (mostly thanks to you folks helping me)
Then I learned (also from you folks) that corys should be in shoals and prefer sand substrate. So, I made a plan to divide my 10 gallon into 2, 20 gallons and make things right. I did my first fish less cycle. After following the guide on this forum, I moved my guppies and panda into the new tank, got 5 new pandas and 2 new guppies. That's when I lost the 2 pandas, one was the one I had in my 10 gallon, and one from then new guys. Tank had been up for about 5 weeks, ammonia 0, nitrites 0, nitrates 10. Was planning a water change the next day but they died before I had the chance.
Sorry for the super long post. But that's my history of fish loss. And where I'm at now.