my goldfish used to gasp for air in their 40g winter home until i added
an air pump. it's a rena brand and is only meant for internal use.

now that the pond is open, i have a waterfall to help with the gas
exchange. i put the rena pump in my pond as well - you cannot have too
much aereation. the rena pump is connected to a bubbler that comes on
from 9pm to 7am every day. i don't like they unnatural way that the
bubbles break the water, that's why i only run it at night. that and
the fact that the plants don't produce any oxygen at night are good
reasons to use and air pump.

my goldfish are all very happy and active and growing like weeds. i
like the way they come over to me and beg for food. ok, time for bed.
don't let the bed trolls bite!