Help to identify the gender of fish...

Question
Hi,
I recently got a gold fish from one of my friend. It pair died as she did not clean the fish bowl for 3 days. I went to a pet shop to buy a pair for the gold fish but the shop keeper lacked sufficient information about the gender. Being impatient to give a pair to the gold fish, I bought one which looked a little different than what I have now. the one that i have now is completely gold in color with white  color at the end of its tail. It doesn't have any white spots on it body. The pair that I bought has whiter tail, looks a little bigger than the other, belly is kind of bigger, but still i couldn't notice any white spots as such. How do I identify if both are females or males or opposite sexes? they both get along with each other. I bought the pair yesterday and kept a watch on the behavior. the new fish eats comparatively lesser than the old fish.
I also bought a new tank with two angel fish and two Mollies. Have the same issue with them in identifying their gender. Both the Mollies look alike... one of the angel fish is more of silverish with black streaks and the other is black with white streaks... can you please help me with this as I'm just a starter with petting fishes and would love to have a lively aquarium in my house...
Sorry if the question typed was too long and took your time. Please advice.
Regards,
Etna

Answer
Hi Etna,
Please write to a Goldfish Expert.  He will tell you everything you need to know about Goldfish.  Goldfish are not my expertise. I'm sorry I cannot help you with your goldfish question.
Angel fish are impossible to  identify male, and female.
The difference between Mollies is that the male as a gonopodium.  A gonopodium is like a little toothpick where his anal fin is.  You must always keep 3 females to one male.  This is because the male will always harass the female, and if there are too many males for the female, she will become very weak, get sick, and die.  Keep in mind that the females will have fry.  They are livebearers, and the pregnant mollie must have lots of room to swim when giving birth.  Never put her into a hatchery, or net.  Keep her in the tank, let her give birth, some will be eaten, some will survive.  This is nature.  Provide silk or real plants for the fry to hide.  They know to stay away from other fish.
Angels, Mollies, and Goldfish do not mix.
Angels need acidic water and a temperature of 82 degrees.
Mollies need alkaline water and a temperature of about 76-78 degrees
Goldfish need alkaline water, and cool water, big tanks are needed for Goldfish.

When buying Angels you must remember that one Angel needs 10 gallons of water all to herself/himself...Two angels need 20 gallons, and so on.
Your tank must cycle or you will lose your fish.  A cycled tank may take anywhere from 4-6  weeks to have the right water chemistry.  This means that your water should have zero ammonia, zero nitrites, and nitrates must be low.  You can have your water checked at the pet store, or you can buy the test kits, and check your water yourself.  These test kits are easy to use, and very handy to have.
To cycle your water, buy a used sponge, squeeze out the access water, and put it directly into your filter.  Let the water from your tank run through it.  A used sponge is full of good bacteria that fish need to survive.  Never rinse your sponge under tap water.  When cleaning your filter, always rinse the sponge in water you have removed from your tank to keep the good bacteria in your sponge.  If you rinse it under tap water, you will kill the good bacteria.  A sponge is good for a very long time, until it is old, ragged, and falling apart.  Let your tank run with no fish in it, adding water that has evaporated.
When adding fish to your tank, you must add them slowly.  Adding too many fish at once would change your water chemistry, and cause death to all of your fish.  Depending on the size of your tank, you can add one, wait one week, do a 25% water change, check your water, and if your water is still perfect add another fish, wait one week, and so on.  This depends on the size of your tank.  If tanks are big enough, you may start with 2 fish.  It also depends on the size of the fish also.
Once your tank has cycled, you must do 25% water changes every week, siphoning from the bottom, always using a good water conditioner such as "Stress Coat" and "AquaPlus."   It is important that you also vary your fish food.  Fish need variation in their diets to get all the vitamins they need to be healthy.  Good water chemistry, and good quality food go a long way in preventing disease.
I hope this helps.
Lynda