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?