This all depends on how big your tank is, how big is it? Two goldfish need a minimum of 20-30 gallons, so if your tank is not big enough, you'll need to upgrade!
Now, the reason why the fish are sitting on the bottom is likely to be stress from bad water quality, the tank is going through a mini-cycling process since you added a new source of waste (the little goldfish), and this is a pointer that you're not providing enough space for your fish since a large enough system shouldn't have such problems. Your biological filter is probably overloaded.
I wouldn't clean the tank, never do that, just siphon out a big part of the water and vacuum the gravel well. I'd change about 75% of it.
You could, and by all means should, buy yourself a test kit so you can test the pH, ammonia, nitrite and nitrate levels or bring a sample of the water to the pet shop and ask them to test it for you
It'd be great if you're post the results here!