Its probably from the filter you are using, and probably from the natural fish oils.
no. they are tropical fish.
like you keep a tropical water fish only you should provide it cold water instead warm water.
It depends on what type of tropical fish you are intending to keep; it ranges from 6 (freshwater & discus) to 8.5 (for cichlids).
Platties are tropical fish and they rely on the temperature of the water to keep their metabolism going. If their water gets too cool, their organs will simply shut down and you will have dead fish. Proper temperature for Platties is about 70F
tropical fish like hard water, with some softness to it, it keeps theyr fins from bunching up and makes them able to swim faster. ( be sure to use a filter to keep the water running)
There are many freshwater tropical fish. In fact most of the tropical fish kept by aquarists are freshwater and they can not survive in either cold water or marine (salt) water. In most cases the marine species are much more difficult to keep and much more expensive to set up for.
you dont
This depends on the kind of fish, if it is a tropical fish, warm water is better. Hot water is not a good idea to use because I am pretty sure it can kill the fish. Most of the time you should use cold water....
Tropical and brackish are not mutually exclusive. Tropical means warm, and brackish means somewhat salty. Many brackish fish ARE tropical, and many tropical fish are brackish. You can keep tropical brackish fish with other tropical brackish fish, if they have the same temperment and will not harm or eat eachother. You can keep some brackish fish with some freshwater tropicals, and similarly you can keep some brackish fish with some marine tropicals. There are no wide open set of principles for brackish fish compatability beyond those that I have just illustrated.
You can definitely do that, but you haven't made it clear that you have cold water in the tank before. If you can keep the water temperature at 72 F, then you can keep both cold water fishes like gold fishes and non-aggressive community tropical fishes like gouramis, platys, mollies, algae eaters etc with gold fish.
That depends more on the fish you are trying to keep. As a general rule most fish can adapt to almost any water conditions as long as it is clean, but not always. Just research the specific fish you are trying to keep to see what pH will work best for it.
Tropical fish can typically survive without a filter in their tank for about 2-3 days, but it is not recommended to keep them in such conditions for an extended period of time as it can lead to poor water quality and stress on the fish.