Why is my fish lying at the bottom of the tank?

Question
Hi
I have 2 gold fish and have recently moved them into a 40 litres big sized tank. Both fish have always spend most of their time lying at the bottom of the tank and they arent active. They dont swim alot.I want to know why this is? are they ill?
They also float backwards sometimes and start to rise to the top of the tank. I feed them flakes and when i do they do come to the top of the tank and eat.They are always together in the tank, lying in the bottom under the filter suction thing.
I would like to know what is wrong with my fish and can their problem be treated?
I was also thinking of getting some more fish but the same type, could i have your advice on this too please?
thanks.

Answer
Hayley,
Yes, I can absolutely tell you why your fish are acting that way. It is their tank. 40 liters is not big enough for one goldfish let alone 2. The absolute minimum size tank for the two you have is 125 liters with a filter for a larger tank. Once you move them to the right size home, you will see a huge difference in the way they are acting. Also, do not feed goldfish flake food. You should be feeding them a sinking pellet food.