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:

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.