I have a simialr problem.

My latest batch of fish have been in the tank and OK for 8-10 weeks.
Suddenly, one was OK and the other hid at the bottom of the tank, would
come up to eat and then go back to the bottom. Last week, it was dead.
The second fish was OK and a few days ago, the same cyccle has started
again. This will be the 5th fish that I've lost this year.

I've had this setup for 10+ years. My first pair of Comets lasted for
about 10 years and now I'm losing fish at a rate of about every 2-3
months.

10g tank with an AquaClear 300. 2 stage1 sponges and 1 stage2 carbon
filter.
1 dwarf snail
1 1 inch corey (he seems happy)
Tettra GF flakes for food.
No gravel, river rocks cover about 30% of the bottom. There is some
green alge on the rocks.
Ammonia, nitrite and nitrate levels are low to zero.
Weekly water changes with some salt and Tetra water conditioner. I have
done more frequent water changes recently hoping to remove whatever is
causing the problem with no luck.

Thanks for any help you can give me.
Ginger