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We have two goldfish in an 18L tank, which has a pump filter and a light. We feed them lightly once a day and I change the water every week or so. We have had them for a few months and they have apparently been doing well overall. (We have plans for a bigger tank, but we wanted to see how it went before investing a lot.)

They have always seemed to prefer hanging out at the bottom of the tank, but recently it seemed that they were both floating at the surface and had to work to move lower down. The water tested okay (KH, GH, pH, Cl2, NO2-, NO3-). I read here that this could be the result of some swim bladder disorder which may be caused by constipation. We followed the instructions there to fast them for 3 days and then feed them mashed pea.

They still are hanging out a lot near the top, and sleep there at night. They don't look as much now like they are floating and don't seem to have difficulty moving around the rest of the tank. After feeding time they do forage along the bottom and all over the tank for quite some time. I noticed they also gulp a lot at the surface after feeding time, even though the food we feed them doesn't float. They always belch air bubbles after doing that, but I wonder if some of the air gets trapped inside.

So back to the fish' new hangout. How likely is it this is a sign of infection? Continued constipation? Air in digestive tract? Also, could the recent drop in temperature account for the change in behavior?

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    Did you test for oxygen? Fish go to the surface for oxygen.
    – paparazzo
    Nov 4, 2015 at 11:46
  • @Frisbee I don't have a way of testing the oxygen directly. Would the fish show any signs of difficulty breathing, like more rapid or vigorous gill movement? I plan to do a partial water change later today, which should increase the O2 level, but won't address an underlying problem, if there is one.
    – Aryeh Leib Taurog
    Nov 4, 2015 at 12:09
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    Are you putting air (bubbles) in the tank?
    – paparazzo
    Nov 4, 2015 at 12:12
  • @Frisbee I do see bubbles where the water flows in from the filter. After poking around on the net a bit I just realized that there is an air inlet on the pump head which we have submerged. It is pretty low down, the instructions were quite vague, and it didn't come with any tubing. I will try to find a tube for that. I imagine that will help.
    – Aryeh Leib Taurog
    Nov 4, 2015 at 12:37

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Fish at the surface is a symptom of not enough oxygen in the water.

If you don't have an air pump injecting bubbles then give that a try.

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  • I've concluded the problem was something else in the water that was interfering with their breathing. We had a little castle in the tank we made out of my kids' clics which must have been releasing some toxin. A few weeks/water changes after I removed the clics, the fish' behavior is mostly back to normal.
    – Aryeh Leib Taurog
    Feb 17, 2016 at 13:52

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