Sucking air from surface

Question
You must get lots of questions about goldfish going to the surface for air, but this case is something strange I believe. I have 6 small goldfish (the biggest is no bigger than 3 inches) since 2 years ago, they were at the beginning in a 10 gallon aquarium and were doing good, after a while I decided to buy a better place for them so now, since 3 or 4 months ago, they live in a 30 gallons aquarium with a big filter and extra air bubbles, natural plants and regular water changes, but despite all this they keep on going to the surface. I tried to measure the acidity levels and they seem to be down to 0, I just don't know what is happening! I don't know if this is related, but 2 of those 6 goldfish are Blackmoores, and they are the ones who more often go to the surface. Could you help me please? I love my goldfish and I have them since 2 years ago. I want them to be as healthy as possible. Thank you very much in advance.

Answer
Marco,
A 30 gallon tank for 6 goldfish is way too small. You are over populated. In a 30 gallon tank you can house 1 goldfish. They are very messy and your fish are swimming in their own waste. They are lacking oxygen. Upgrade them to a minimum size tank of 80 gallons. Make sure the filter is for a larger tank. I am thinking your air stones are not large enough. If you want these fish to survive, get them a larger home and add adequate oxygen to the water.