You could transfer some of the water, but I think transferring the brown gunk from the filter in the goldfish tank to the new tank is a better idea. That stuff is way more loaded with the good bacteria you want. So after you do that, wait at least a few days, and check the ammonia levels every day. And don't go too crazy with water changes during this time. You have a small tank... the idea here is to give the tank a chance for bacteria to grow. If you change out too much water too frequently, you'll be slowing that process - or preventing it all together. You want the ammonia to go down -- but because there's adequate bacteria in the tank to handle the waste, NOT because you're changing out so much water so frequently. Hope that makes sense. Keep us updated!