Welcome to fishlore ruthiesmom:
Your tank may be overstocked and adding the pleco's biological load could have surpassed the capacity of your bacterial colonies to cope with it.
I see in your profile you answered NO to the nitrogen cycle question. Please take some time to read it (first time I ever heard of such thing was here in Fishlore; getting to know this information has allowed me to successfully keep 7 tanks runnings)
Please check it out in the following link:
http://www.fishlore.com/Articles/TheNitrogenCycle.htm
Make yourself at home
Pepe
Santo Domingo
PS We love pics here. Hope you can upload a few of your tank for us.