fish seem to gulp underwater all the time - is it normal

Hi,

My girlfriend is new to keeping fish and has recently brought a 35
litre tank. She set it all up except the fish and one week later
added 2 goldfish, maybe 1.5 inches in length. Then one week later
still she added the final two fish which are about 1inch. So she
thinks she has done everything correct.

The only thing she is not sure is right with the fish is that their
mouth constantly gulp. This is underwater and not at the surface.

They all swim around and look for food near the bottom (she uses
sinkng food) and in the evenings they seem to get more active and swim
around mind tank and also near the surface sometimes. They seem
pretty active.

Their mouths open and shut all the time. From memory she doesnt think
her friends fish mouths move so much and I have no idea myself.

The ones in the shop dont but then they are pumping oxygen into the
water all the time, whereas my girfriend just has about 4 plants and a
pump to move the water around (plus the filter etc).

So is is normal for the fishes mouth to move this much.

Sorry if this is a really stupid question.

Regards,
Scott