Can't keep my Corys alive

[Maersksins]

I have a 2.5 gallon quarantine tank with a small filter and heater setup to bring in new fish to my other tanks a few at a time. This past week i put 3 small guppies and 3 small Julii Corydoras in, but 2 corys have perished. I have had about 10 corys die on me over the past year. As of now i only have 3 surviving of 13. My water temp is 72° (a bit lower than i wanted) and the Ph is around 7.8, and the water here is a little harder ~240 GH ppm. Not quite sure what i am doing wrong here. Anybody have any advice?
First time owning Corys and it so frustrating to hear how easy they are to keep but i can only seem to keep acouple alive. i just started getting back into the hobby after being out for many years. I feel it is either the hardness or temperature of the water.

 

[Dallascowboys16]

I have a 2.5 gallon quarantine tank with a small filter and heater setup to bring in new fish to my other tanks a few at a time. This past week i put 3 small guppies and 3 small Julii Corydoras in, but 2 corys have perished. I have had about 10 corys die on me over the past year. As of now i only have 3 surviving of 13. My water temp is 72° (a bit lower than i wanted) and the Ph is around 7.8, and the water here is a little harder ~240 GH ppm. Not quite sure what i am doing wrong here. Anybody have any advice?
First time owning Corys and it so frustrating to hear how easy they are to keep but i can only seem to keep acouple alive. i just started getting back into the hobby after being out for many years. I feel it is either the hardness or temperature of the water.

For starters, the 2.5 gallon quarantine tank is most likely stressing the corys out. Aside from being quite small for most Cory species, it likely suffers from high buildups of nitrate and other waste products unless you are changing the water in it every 4 or 5 days. The temp is also probably stressing them. If I had to guess, the low temp combined with the very small QT tank is likely doing them in. Most corys will adjust to a range of hardness within reason so I doubt that is the driving issue. Are you also losing fish in your display tank or is it just fish in the the QT setup?

 

[Maersksins]

I had lost a couple in my 20G planted, which is my primary tank, due to varying issues but has since leveled out and become stable. They seemed to be a lot more skiddish and active in the 2.5. I really want to have a nice school of corys maybe 6 or 7 so i think i will try to put them straight in my 20G without quarantine and just deal with any issues after they survive a bit longer.