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I used to have a very small fish tank (can be seen in this older question), and finally upgraded it to much bigger tank, moving the fish into it.

In the old, small tank, the fish used to hover a lot, seeking food, and consumed the grains almost as soon as they were given to them. However now in the new tank, they don't reach the top anymore, and stay deep inside the water most of the time.

The grains of food float on the surface and the fish don't reach upwards to get them even hours later:

fish tank with grains floating

You can see the grains of food floating in the front left, and the fish far below.

Is there any way to "lure" the fish to swim upwards and eat their food? Or different type of food that won't float? As far as I could see, goldfish food is only in the form of grains, which always float.

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    What are the water parameters of the tank? – SerenaT Mar 24 at 11:55
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    Ammonia, nitrite etc. – SerenaT Mar 24 at 13:57
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    No appetite and lethargy in a fish like goldfish is a sign that something is at least moderately wrong, if not seriously wrong with its health. Please measure the concentration of ammonia, nitrites, pH and nitrates and include the results in your question. Please include the information: how have you sanitized the water before filling aquarium with it? Did you use water conditioner with dechlorinator, did you use tap water or well water or reverse osmosis water? Could you include some close-up photos of the goldfish, especially the gills? Are the gills bright-red, tan-brown, or else? Thanks. – lila Mar 24 at 20:05
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    If you dumped out the water from the old tank and filled the new tank with fresh water, the fish are currently poisening themselves. They have no toilet to discard waste, the waste gets absorbed into the water and poisons the fish. Please read this question for more information. If you have any neighbor or friend with a fish tank, ask them for some water from their tank to kickstart your new tank. Some pet shops also sell cycled water or aquarium starters. – Elmy Mar 24 at 20:08
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    And stop feeding your fish right now or the surplus food increases the problem even more. Your fish won't starve if they get less food for a few days, but they will die of ammonia poisoning if you doin't act very quickly now. – Elmy Mar 24 at 20:11
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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.

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    I have to add that transferring the water from cycled tank is not that efficient because nitrifying bacteria adhere to surfaces and are not that abundant in water column, it is definitely better to transfer water from a cycled tank than do nothing, but even better is to transfer some substrate, filter media and plants from a cycled aquarium. – lila Mar 25 at 12:11

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