One of my mollies has been staying at the surface and seems to be gasping

[Essjay]

It depends on your pH. If it is well below 7.0, that's OK. If it's 7.0 or above it is not OK and you need to do a water change.

Ammonia exists in two forms in water, ammonia and ammonium. Ammonia is toxic, ammonium is a lot less toxic. Our test kits measure both combined so just by the reading it is impossible to tell how much is in the toxic form.
The amount in each form depends on the temperature and pH. Since temperature doesn't vary much in tropical tanks we can ignore it. But pH varies from place to place. The lower the pH, the less that is in the toxic form; the higher the pH the more that is in the toxic form.


Can you tell us the pH of your tank water please - there are calculators we can use to work out the amount of the toxic form from the pH.

 

[Goose3080]

Just too add to this, try doing an oxygen test too, I lost a fish due to lack of oxygen in the water, make sure you take the water sample from the bottom of the tank and then test, pets at home will test your water for free, just take about 30ml into them and they will test it, normally fish come to the surface and gasp for oxygen, the fastest way I've found to get oxygen into the tank is with a water change.

A lot of people think that more bubbles being generated creates more oxygen, but this isn't true, it just looks good, oxygen is created by agitation on the water surface, think motion of the ocean, they don't have bubble machines under the surface, the waves create the oxygen.