Hi, I am a newbie to fish and I desperately need some help.

I have a 100 gallon tank with 2 types of pump filters, one that gives
white powers stuff, and one that contains black carbon filter. I got
these from a friend and he helped me setup everything. He use to do
this a lot but has no room for it anymore. He use to have salt water
fish but said frehwater goldfish is the easieste...

We put water in and connected everything. Put new filter/carbon into
both filters, also installed those bubble thing at the bottom that
helps oxygen. I got so excited, I decided to buy some fish. So I got
about 10 small goldfishes and put it in the tank. (actually we put the
plastic bag that contains the fish/water into the tank and let it sat
for about half an hour. He said it helps the fish to adjust to the new
water temp.

Then we opened the bag and put the fish in. everything looked good. I
even put a bit of fish food into the tank. The fish is a little shy
but one of them came up and ate some of it. but after a day or so, the
fish begin to look not so good, and today all of them have died. They
are definitely not short of food, as there are still some food
particles at the top of the water. (actually i wasn't home yesterday
so they could've died yesterday. i don't know.

I am puzzled, what could it be? Here are some of the things I can
think of.

1. water temperature is too cold.

It has been pretty cold here in the past few days. I don't know what
the water temperature is, but it is slightly cold to the tough (but
definitely not freezing). I'd estimate it to be about ~45-50f. The
water is from the tap. I am in the bay area, the water here is
slightly hard, but reasonable.

Some GF tolerate temperatures below 60F but others do not. You may have
gotten some GF that just couldn't take such a low temperature.

2. Dust/dirt from old equipment. Although the water looks very clean.

Did you treat / dechlorinate the water? If your tap water has lots of
chlorine / chloramine in it that will hurt the fish. My first guess when I
read your post is that you used straight untreated tap water and the
chlorine / chloramine got them. Was the tank cleaned before you got it? If
it (or any component in the tank) was cleaned with bleach or other household
cleaners it must be rinsed and rinsed and rinsed - even a tiny amount of
bleach is a real fish killer.

3. Do I need to keep the lamp on 24 hours a day? Does the fish
care/mind if the lamp is off at night?

Both fish and plants appreciate a regular day / night cycle. I turn my
lights on in the morning and off at night. I really should really get a
timer but it's so much fun to turn on the lights myself and have the fish
rush to me begging for their morning feed.

The thing is, about an hour after I put the fish in, my friend said
some of them doesn't look too good, about 4 all sit at the bottom
resting... a few of them was still swimming around.

You could have had sick fish to begin with, but that sounds like chlorine /
chloramine in the water to me, perhaps compounded by temperature shock if
the water was that cold.

I am going to ask my friend about it tomorrow, but I feel bad because
all of them has died for some reason...

Any help will be greatly appreciated.

Treat and salt the water (GF do well with a bit of aquarium salt in the
water - I usually add half the amount directed on the package) and let it
sit at least a day before adding fish. Get a heater (for a 100 gallon tank
you'll need a big one) and keep the temp at about 68F or so - most GF do
fine at that temperature (if you decide to go with warm-water tropicals they
need about 78F or so but GF are cool-water fish; don't mix cool-water GF and
warm-water tropicals). Read about "cycling" and make sure your tank is fully
cycled; get a test kit for ammonia, nitrates, and nitrites and learn how to
use it. A planted tank with a gravel bed helps maintain a stable
environment, but some GF are notorious plant-eaters and plant-uprooters. GF
are also notoriously dirty and need agressive filtering and regular water
changes. GF need lots of aeration (sounds like you already have an
airstone - one isn't enough for a large tank) and like to have a fairly
strong current in the water; I use a powerhead on an UGF for filtration (in
conjunction with a cannister filter, which you seem to have already) and
current in my GF tanks. Read these ngs where lots of friendly folks will be
glad to help you out (while lusting for your 100-gallon setup

--
John Goulden
mainly goldies, guppies, swordtails, and bettas