One possibility is that may be causing the fish to gasp at the surface may be that the water does not have sufficient oxygen, which is often the case during the warm days of summer and the water warms up, please note that as the water warms up then the actual oxygen level drops and therefore it is essential that you do provide an alternative air source for them ...  the oxygen may also drop when you kill off a bunch of algae or snails.

Decaying uneaten food, plant material, etc. can consume massive amounts of oxygen.

Also when you treat your ponds with Formalin please be aware that this also consumes oxygen at a high rate so always ensure when you are treating your pond that you do have plenty of oxygen.

Formalin is contained in many of the medications that are available to the hobbyist - but a word of warning Formalin should never be used if the pond has been salted ... as this can cause a chemical reaction which may severely harm the fish ....

There are many good Air Pumps available on today's market and we have an excellent choice on our web site ... (air pumps)

Having checked your oxygen level and decided that your level is OK - then what other reason can there be to cause the fish to gasp at the water surface ..

It may well be some other cause like some gill pathology, or even like a pH crash, parasites, over treatment with medications, High Chlorine levels, and other caustic stress to the gills may be actually causing the decrease in your oxygen levels.

So Increase the circulation of the water in the pond, if the pump you are using does not have sufficient power then change it for one that does have the power that you need and you can if you wish fit a good venturi . this will automatically increase the oxygen level in your pond ... but please bear in mind that if you are going to use a venturi that you may need to consider the size of your pump as a venturi does take quite a lot of pressure to make them work effectively. 

We have a superb venturi that is easy to fit and does not require as much power as some on the market you can see these Venturi's on our web site ...  see air master