Pleco... It is that hardy?

Last year I bought a total of 5 pleco. (2 were bought by my mother but influenced by me). All of them died within a month. They manage to live under me for about 2 - 3 weeks. No injuries or diseases were seen . Its tank mates were 4 firemouths (juvenile) and one adult krib.

My New Fire Mouths

I bought 4 fire mouths last year. 2 died while i am in college. Than during my break I bought 2 more. I still cannot figure out which one is male or female. The 2 survivors are a lot bigger than the newbies. The largest usually chase the others for no good reason ( dominant feeling). I am hoping it will start breeding this year.