No, fish can't hear.
totally in tanks I've had a fish in a tank and in a bowl and the tank ones lived longer
If the goldfish is a fancy variety, it can live alone in a 20 gallon tank. Goldfish do not belong in tanks any smaller than this as they grow very large
Yes, the bigger tank the better. They actually HATE goldfish bowls. (they're not nearly big enough). Goldfish usually need a 20 to 30 gallon tank
It is much better for the ifsh if you add a pump or filter in your tank.
Wouldn't recommend putting a goldfish in a 5 gallon tank-regardless of what type or size of goldfish. This is because the goldfish would be stunted causing it to have an untimely death. The condition in which a goldfish is "stunted" is where the fish adjusts to its tiny environment so much as to stop growing. The fish will stop growing on the outside but will continue to grow on the inside. Goldfish are naturally messy and create a LOT of waste. This factor would cause the tanks water to be poluted which inturn could kill not only the goldfish but possible your mollies too :(. Two mollies is enough for a five gallon tank. If you truly want to get a goldfish you will have to house the goldfish in a 10-20 gallon tank with very good filtration
A goldfish can live in an opaque glass tank as long as you keep up with necessary water changes. Like a small outside garden pond goldfish are only viewed from the top.
Although there are likely no studies on this, from my personal experience the noise doesn't appear to bother them at all. Since the pump is outside the tank, anything they might hear is probably quite muffled.
A properly working filter can really improve the living conditions for your goldfish and will almost double the tanks holding capacity. The tank you have though is not large enough for any kind of goldfish. 1" of fish needs 1 gallon of water. Your goldfish will grow to around 10" if given reasonable conditions. That is going to max out your tank with just 1 fish.
You cant. Goldfish need much bigger tanks. 20-30 gallons. I bought 2 goldfish when I was new to aquarium hobby and put them in a 5 Gallon tank. The first one died in just one and a half day. The other one died just hours after the first one :( So if I were you, I would get a bigger tank.
Tanks that are reffered to as "breeder tanks" work very nice as a goldfish aquarium. They are quite wide and noticeably shorter than the average tank which is more of a rectangle and doesn't permit as much gas exchange. An aquarium that is wider has a larger surface area thus allowing more gas exchange resulting in higher dissolved oxygen levels.
Yes
Goldfish may be kept in an uheated fresh-water aquarium. They are considered ill-suited to live in a heated tropical fish tank, as they are used to the greater amount of oxygen in unheated tanks.