When fish gasp at the surface, it generally follows that they are short of oxygen. Often this is due to other pollutants such as ammonia or nitrite interfering with healthy gill function in the same way that we might fight for breath in a smoky atmosphere or in the presence of noxious fumes. Another problem that can cause the same symptoms is a pH crash and the signs of Acidosis are very similar. So, as we can see, checking water quality parameters should be the first course of action. High temperatures affect the amount of oxygen available and so heating equipment and aquarium temperature should also be scrutinised.
If all water parameters fall within healthy levels, other factors can be reviewed. The presence of large amounts of plants or algae in a system can drop the oxygen levels to a critical point in summer and in heavily planted ponds or aquaria some overnight aeration or extra water movement may be necessary to drive off carbon dioxide and increase dissolved oxygen content.
An over-stocked aquarium or pond can suffer low levels of oxygen due to the metabolic requirements of both fishes and filter bacteria. If your system needs extra aeration to get by, it may be time to review your stocking levels and treat yourself to the larger aquarium or pond you’ve been considering!
There are a few species of fish that will naturally feed by skimming the surface film, such as some livebearers. By observing your fish for signs of distress, you can see what constitutes normal behaviour for your pets.