Seem to be doing everything right... fish keep dying

I've just purchased and setup my second aquarium 20 (canadian)
gallons. The setup went smoothly and I had the tank running for about
two days with some new plants and simple decorations for 48 hours
before introducing any fish.

I installed an undergravel filter and a fluval 150 for the filtering
piece, as well as a heater for temp regulation etc...

In my passed experience (had a 10 gallon), I was really succesful with
this same setup and one of the first things (upon recommendation) to
get my cycle going more quickly was to steal some of the water and
bottom bacteria from the Pet Store (they supplied a big bag), and
stock the aquarium with some cheap feeder fish.

This time, with the exception of adding real plants, I followed the
exact same course of action. I started with about 15 (really small)
feeder goldfish, some tank water from a healthy pet store tank, and of
course my live plants.

I've been monitoring the PH (around 7) and Ammonia levels daily
(practically none) and have kept the tank a steady 75 degrees, however
my fish KEEP ON DYING.

At first, I thought this normal as feeders are usually unhealthy and
overstocked to begin with, however everyday since purchase I've had on
average 2-3 deaths. The fish seem healthy enough, eating swimming,
high energy, however they keep on dying slowly..... one at a time.

I've bought every test I can think of to check the water... could it
be something related to all the new equipment which may not have been
fully rinsed ? Would the fish not be showing clear signs of desease
before expiring if that were the case? Could the plants I put in be
having some kind of effect.

thanks to anyone who could help with this???