Firstly, your 10 gallon was overstocked. 10 gallons per goldfish is the minimum
as goldfish are
extremely messy fish. Secondly it sounds like you did not cycle your tank. You
need to cycle
the tank to establish good bacteria that breaks waste ammonia into nitrite into
nitrates (do a google
search!!)
Ammonia and NitrIte can be lethal to fish, Nitrate is not as bad as long as it
is kept below 20ppm
(some people say below 40ppm). Thirdly, it sounds like you did not quarantine
your pleco - always
quarantine new species before putting into a main tank as they can brind
diseases, lastly Plecos and
goldfish do not mix. Plecos develop a taste for slimecoats as they get older.
Thats is my 2 cents!