Ok, so I've got three fish in a 30 gallon, with ample filtration and aeration and water changes once a week. The parameters are:-

Ammonia 0
Nitrate 0
Nitrite 20
pH 7.6

But the fish seem quite listless. In the morning when I turn the light on they get quite active, but when I get in from work they're usually either sitting in the bottom or hiding quite still in the plants. Then, when I turn the lights off they get active again.

I've started treating for internal bacteria, but don't know what else to do. Can anyone shed any light on this?

I don't understand cycling very well but having 0 nitrates sounds like the tank is not cycled.

This link might be useful. http://www.kokosgoldfish.com/cycle.html

Edit: It is probably not the cycle, it's just a theory.

I agree with Lumpy. You should have nitrates but no nitrites. If the reading it true, youll want to do a big waterchange right away. Im guessing it's just an error though. How long has the tank been running and what kind of filter are you using?

Sorry that was a mistake. Should read:

Nitrate 20
Nitrite 0

kH & gH?
Ph Level out of the Tap?
What is the name and size of the filter/s?
How often do you change the water and how much?
Temperature? Does it ever fluctuate?

Do they respond when they see you come in or do they seem to "freak out" and try to get away from you?

Great questions,and I also wanted to ask you Brock,how long the tanks been running/how long have you had the fish,and were they all quarrantined before added to your tank?
Are you seeing any clamped fins at all,
How strong are your lights and how many hours do you leave it on?Do they make any huge difference on tank temps when you're using them?