It's typically not recommended. Goldfish are cold water fish, and are used to the levels of dissolved oxygen in colder waters. Since warmer water holds less dissolved oxygen than cold water, it may negatively impact the health of your goldfish.
Yes
It is important to remember to put things in a goldfish tank, even goldfish get bored. Some of my suggestions for tank decor are artificial plants and a "hidey hole" for your goldfish.
You should not have goldfish and platies in the same tank ever.
depends, can a ten gallon tank live in a goldfish
getting more goldfish in your tank
Regardless of the type or size of goldfish the minimum acceptable size of an aquarium for a goldfish is 30 gallons. Buy a tank that is at least 30 gallons if you plan to have goldfish. You can comfortably fit 3 goldfish in a tank this size but no goldfish in a tank any smaller. Goldfish are essentially pond fish and need lots of space to swim; they will not thrive in a small aquarium.
no. you should have 10gal of water per goldfish. you shouldn't have goldfish in a 5gal tank at all. they are huge waste producers
Either a tank or pond of at least 30 gallons is a suitable environment for a goldfish.
Yes, goldfish are very messy. Goldfish need 10 gallons per fish, you actually need a 20 gallon tank or about a 70 liter tank for 2 goldfish. But the small tank will do while they are small, you will need to eventually get a larger tank.
No. A one gallon tank is not appropriate for a singular goldfish much less multiple. A ten gallon tank is the bare minimum for a singular goldfish and breeding would require a tank of twenty gallons or more. A one gallon tank cannot handle the chasing that proceeds goldfish mating nor maintain a water quality that is suitable for raising goldfish fry.
glass tank
Yes, but your tank size needs to support it. If you mean that a regular goldfish is a long single-tail goldfish, then you need at least a 40 gallon tank just for the 2 fish due to the size potential of the common goldfish. If you mean a regular goldfish as in a round bodied fantail, or perhaps a ryukin, then you need at least a 30 gallon tank for the 2 fish.