To put all the comments into an answer: You probably forgot to cycle your tank. That's a mistake many fish owners do because pet shops neglect to tell them about it.
What "cycling" is and how it works is described in this very detailed answer: How does one effectively “cycle” an aquarium?.
Fish becoming lethargic and either sinking to the bottom or gasping at the surface of the water is a sign that they feel ill. The most likely reason why they feel ill is that there's something toxic in the water. What exactly that something toxic is can be measured with aquarium water tests. The good news is that your new tank is big enough for your fish. The water volume can buffer a certain amount of toxins and makes partial waterchanges easier.
The second reason for this behavior is shock. If you take fish out of their usual water and dump them into completely different water, they need time to adapt. That's another reason why you should transfer water from your old tank to the new one. For more information, please read this question.
Since the cycling of water is done by bacteria, a first aid measure for an uncycled tank is to add either some water or an old filter medium from an already cycled tank. That "transplants" the beneficial bacteria into your new tank. Many pet shops also sell cycled water or "aquarium starters".
If none of those solutions are available to you, you need to lower the toxin levels in your water by doing partial water changes each day until the tank is cycled. Don't change more than 20% of the water volume at once to avoid shocking your fish again.
If your tap water is chlorinated, you absolutely must dechlorinate it before putting it in the tank. The chlorination is meant to kill bacteria in the water, but it also causes chemical burns on the gills of fish and suffocates them.
Since you seem to have no living plants in your aquarium, you may get too many algae in the future. There's no need to panic now, I just want to make you aware of it in case it happens. Please read this question for more information.