Here's the deal: I have, the last two months, noticed that one of my fish is hovering by the surface, no gasping, no nothing. Just hovering motionless (which I read would be because of low pH). About two weeks ago, my second fish started acting in a similar way, both of them decide to swim around normally most of the time, but they stay hovering like this for 5-15 minutes. My third fish is perfectly okay and has never displayed this behavior. So it doesn't seem like it'd be caused by water quality problems imo. Nor swim bladder disorder because they can easily control themselves.

I change the water once a week (33% of it), add water conditioner and bio filter booster. I test the pH 2-3 times a week (it's on 7.5) along with the KH, I check the nitrite levels about once every two weeks and it's always 0. I am intending to get a nitrate and ammonia test as well, just to be on the safe side. My tank is, as of now 8 gallons, three small fish at 1.5 inch each. It has a sponge filter and an air stone.
I've been reading in all kinds of books, checking on disease, possible water problems... but nothing seems to answer my questions. I even examined the gills which looked meaty and healthy as they should!
I feed them shelled peas every day, along with pellets (both sinking and floating). And freeze-dried live foods twice a week.

Is this hovering something to be worried about? And if that's the case, what do I do?

I see! That may be why they are more active once the water is changed. Thanks (:
I'll try to repair the problem asap!

Thanks! It's nice to hear that (: Since I maybe don't have the best possible tank for them (yet) I decided to try and take as good care as possible of them as it is. Namely by looking closely at them every day and getting to know them (:

Can you increase aeration in your orb by turning up the air pump? If not then I would create more surface air by decreasing the water level at least.  Remember, the more water surface a tank provides, the more oxygen will be in the water and the more fish, the more oxygen they will need.  3 fish in an 8 gallon orb would be really difficult to to maintain oxygen levels I would think.  Goldfish are oxygen hogs.  Also, higher water temperature mean less availble oxygen too to a tank.  Do you heat your orb?