Maybe it's too cold for him? Some fish are more sensitive to temperature than others. Maybe you should get a heater for him in the winter.

Tighebettalover

What kind of set up do you have for the fish when you bring them in? And is there any way you can keep your pond fish outside during the winter? They're being moved from what size pond to what size indoor receptacle?

I finally made the move to dig my pond deeper to allow me to keep my pond fish outdoors all year round - and it's really less stessful on the fish because I'm not having to chase the fish to catch them or empty the pond to make sure I get them all. My pond is about 1000 gallons.

It sounds like he may have some sort of on-going bacterial problem - probably affecting the swim bladder - and when he goes back into the less stressful more appropriate accomodation of the pond, he grows strong enough to fight the infection. Some fish may just be more prone to infections as is with humans.

Tighe - comets and koi are cold water fish. They can be kept outdoors in the winter in a pond as long as the water is deep enough not to freeze throughout and there is a hole kept open in the top ice to allow gas exchange. Koi and comets go into winter hibernation.

Thanks much.

I also thought it might be some sort of bacterial problem.

The aquarium they go into for winter is much smaller - around 40 gal. with an Auto-Flo filter system (the kind that hangs on the outside of the tank). All the fish are around 6 inches in length.

I'm not sure of the pond size in gallons, but it's around 4 feet across and 18 inches deep. Since it is a preformed shell, I can't make it deeper. I also live in MN, so winters can get very cold.

Since this problem doesn't show up until half way through winter, is there something else I could do? Medication? I've also thought maybe their winter diet isn't as healthful or varied as it is outside with all the stuff they find on their own.

Kevin

First - consider getting a much larger home for them in the winter. Couple of alternatives:

Try and find the rubbermaid stock tank. They come in various sizes. 40 gallons is really super small. How many comets do you have?

What I suggest you do is try the pond forum on this site and pose this question. You may get more responses - also more ideas on how you can better do a winter set up. There are folks there who live in much colder climes but they keep their fish in the pond all winter long. It's just a matter of setting up the pond for the winter.

I live in zone 7 - so except for occasional super cold winters, most of the time I can keep my pond ice free with a pond heater. I also keep the waterfall going (primarily for the wildbirds who use the pond for their water source during the very cold winter months) - although some ponders don't think this is a good idea. But I've been ponding for nearly 2 decades now, and I haven't had any issues. I did dig my pond in one spot down to 4 feet. But it slope up and there are shallow spots.

Being in zone 5 is really not that cold for carp if you can dig your pond deep enough. You're right though, the preform is too shallow for an all year pond.

What's probably happening is the cramped space and lack of really good filtering is what's affecting that comet. When I had my indoor set up (set up in an unheated enclosed porch where even in frigid weather never got colder than 35 degrees since it faces southeast and gets good winter sun to warm it up) I used the same type of biofilter set up that I had in the pond. At that time I used a beckett filter. I also hooked up a marineland biofilter on the side. The receptacle I used to hold the fish was a 60 gallon preform - but at that time I only had about 5 fish - 2 koi and 3 shubunkin. The largest at that time was only about 8 inches long. When I started acquiring more fish and some babies were born, I realized I needed to enlarge my pond so it could weather winter. The indoor set up was a constant chore. I was constantly cleaning the filters because of the heavy bioload. The fish were still somewhat active since the water temps really never fell below 50 degrees - so they got fed during the winter as well.

What you may want to try is medicated food - to see if you can knock out the bacterial infection - but if he's not really exhibiting any signs of illness, I wouldn't do this. The medicated food I've used before (indoors with a goldfish tank) seemed to cloudy up the water.

Do go and visit the pond forum - you're bound to have more suggestions and advice at the board since it covers more that's specific to your query than here.