Well I don't really know exactly what it is but there is one thing you can try to do that MIGHT help and won't hurt a bit if you do it.
You can try to feed the fish a bit of frozen and thawed peas. You fast the fish which he is taking care of for you and then fix one frozen pea by putting it in the microwave on a saucer (no water) for 30 seconds. Skin the outer skin off the pea and take a chunk of pea half the size of the fish's eye and cut in 3 pieces and offer them to the fish. If he is constipated, this should be a help. The symptoms you are talking about could be signs of constipation and probably indicate the fish is eating too much. If it continues, the fish can get swimbladder disorder and will start to be unable to swim correctly and will shimmy in the water.
This is the first course of treatment and as I said will not hurt anything but will probably have to be repeated several times to clear the fish's system.
Please let us know how he is doing.
Rose
