Personally I wouldn't put clay in a fish tank due to the properties of clay. It will change the pH of your water and as the clay sits in the tank it will soften and dissolve clogging your filter an clouding the water. In addition, to many of those tiny particles floating in the water can't be good for your fish' gills either.
Just an opinion.
Fish Tanks.
cat fish eels peranas (take it from a person who has 5 big fish tanks with 150 fish)
It depends on what kind of plants in the fish bowl and what kind of fish. Pet stores sell decorative plastic plants as well as aquarium plants (real plants) for fish tanks. I think that there are even certain types of garden plants that may work in fish tanks, but that requires more research to make sure it is safe for your fish.
Not as such. But UV light can help epipelagic fish (surface-dwelling; the kind usually found in fish tanks) maintain their natural coloration.
good fish good fish good fish
Floran fish can not live with many other types of fish. They are highly aggressive and should be kept in species-specific tanks with their own kind.
Soil is not used in fish tanks, gravel is, it comes in many different sizes and colors. Alternatively, you can use sand (make sure you get specific aquarium sand so you don't end up introducing harmful substances to your fish). They also make plant mats that can be used and are good for breeding fish.
it has water pollution because of it's sewage, septic tanks, and cesspits. They also over fish in the Keys.
They can usually get along with most fish but sometimes turn aggressive when they get older especially when they live in small tanks. See sources and related links(below).
Any.
They are OK with most Livebearers, Barbs and Tetras.
you can use modeling clay, well at least that is what i used