My goldfish keeps sleeping in the plants and it looks like he's dead till I knock on the glass and he suddenly moves again?

Good on yer. Keep knocking on the glass and eventually you will kill your fish. Each time you knock on the glass you are sending percussion shock waves through the water and you are hurting your fish badly. If your fish is lethargic I would guess there is not enough oxygen in the water for the fish to be active. This is usually because the tank is too small and/or the water has no filter. The basic rules for successfull fishkeeping are :- 1 inch (2.25cm) of fish needs at least 1 gallon (3.7 ltrs) of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water replaced every week. They are the basics and then each species of fish has its own special needs when it comes to temperature, pH, GH and type of food. If you provide all the above your fish stand a chance. If you don't I can guarantee that you will have constant problems with the health of your fish.