48kg per hour
Goldfish excrete, yes.
Yes
Through their anus, same as you.
About 3 and a half kilograms
Tetras (unlike goldfish), do not excrete a lot, so in essence they would be "clean" fish.
Under normal circumstances, it is best not to mix goldfish with other fish. They excrete higher amounts of ammonia, and it is harsh on the other fish in the tank.
There will be a problem either with the water or the diet. My advice is to do a few large water changes taking all the proper precautions and equalising all parameters. Then I would improve the fishes diet by adding a feed of earthworms at least once a week.
That's pretty much the traditional goldfish.
I would guess 367
£1.29 for ordinary Goldfish and £2.99 for any Oranda
too much
A goldfish should eat as much flakes or pellets as it can in 2 to 3 minutes twice a day.