I've had a thirty gallon aquarium for some fifteen years, and I never change the water except for partial changes, like you do in a pond, about 1/4 of the water every two weeks. There should be an undergravel filter included in your kit, although some people question the effectiveness of it. It does help the biological bed in the gravel, though, so be sure to get a small bottle of some kind of beneficial bacteria. You should probably get a gravel vacuum, a type of syphon that sucks up fish poop and other debris, and use it for your water changes. There might be one included in your kit, so be sure to check before you buy one. Two comets should do just fine in a 29 gallon tank.

Rubyshoes, even in Chicago, I don't think your pond will freeze all the way to the bottom although you might have a really cold microclimate. Your fish are certainly large enough to survive so long as there is a hole in the ice for gas exchange. Read some more of the threads about over wintering. I know that the recommendation is for three foot depth but several forum members have succeeded with less depth in your area.
On the other hand, I enjoy my aquariums in the winter. I use a Python siphon to change the water and get rid of gunk. It lets me keep the aquarium clean without hurting my back. It connects to a water faucet that is almost 20 feet away. The fish don't panic as the water gets lower during the change and the water that flows in mixes with the old water nicely. I do add a dechlor chemical since some fish are very sensitive to chlorine in tap water. I have been seeing some filters that include bacteria but usually there is a normal buildup of bacteria in the gravel and filter material. No harm in adding bacteria in a bottle though. Sandy

Thank you both for the replies. I'm probably going to go with the aquarium. I'm just too afraid to leave the fish in the pond. I never was much of a fish person but I've grown very fond of my two comets! My first thought was to just bring all the pond plants in and recreate the pond inside. I have access to a large stock tank for free. But after the initial excitement of thinking about it, I realized we just do not have the room.

I'll look into the Python siphon also. I guess I'll just have to do a lot of reading up on aquarium care.

I must confess that when I started my pond I just wanted to hear the sound of water in my garden and put a few water plants in it. But the next day after I filled it (back in early July) the water was crystal clear so I decided to get some fish, which I proceeded to just dump into the pond the next day. I must have just been lucky. The fish are thriving but the water is murky in the last couple of weeks. So now I have to keep reading up on that along with learning about aquariums and learning about pond maintainence for the winter, etc. etc.! What have I gotten myself into!??? But I've gotten loads of enjoyment out of the pond and the fish so I must be "hooked!"

With goldfish, you do not want an undergravel filter. They generate a lot of waste. A good external filter like an Aquaclear or something similar with a high flow rate is what you want. You have to be careful to clean the filter and change about 25% of the water every two weeks or so or else the wastes can build up fast. If you try plants, the goldfish will eat them. You might be better off going to an aquarium store and talking to them to see if you really want a kit or will it be cheaper to get the right things intended for what you need.

Kalevi, thanks for responding. I think I will check into getting the separate components. I was reading up on the Aquaclear filter the other day. So, all I'm sure about now is that I am going with a 29 gal. tank.

I use a 90 gal and a 55 gal to overwinter my fish. The 90 has a big expensive filter from when it had tropicals in it. The 55 on the other hand just has water in it (g). No light, no heater, no gravel. For filtration I have a small pond pump with attached filter that does the job quite well. Partial water change every couple of weeks and clean the filter every now and then. Works well. Using this type of filter allows you to keep the water level down a few inches from the top so they have a harder time jumping out. And a "just a couple of fish" always seem to turn into a "couple more fish" so you might end up needing the extra room the 55 will give you...john

Ruby, we always bring a few of the goldfish in for the winter just for fun. We have a 36 gallon tank, and like everybody says, a 25% water change every other week works for us. We have a python as well. Got it at the local fish store. We have an external pump with filter pads (an Emperor, I think) that sucks the water up from across the bottom. It works well. Then I rinse the little stones real well in the spring when we put the little fishies back into the pond, clean the whole set-up, and store it until fall.

After the first year we got plastic plants because they keep eating the real plants. But I buy them watercress at the grocery store once in a while, and sometimes I throw in a few frozen peas (skins removed).

They're so much fun to watch when all you see outside is snow!

:) Mary

*and watch out... it gets worse over the years... we now also sport an indoor preform pond, supposedly for the tropicals from the outdoor pond... with just a few fishies as well... and of course that needed a mini skippy*
*and an aquarium heater and lights and houseplants*
*the things we do for the sound of dripping water*

Last year I bought the plants cambomba and hornwort when I brought in a 6" fish to overwinter (brought it in because it had fungus). The goldfish loved to munch on the cambomba, which I figured was fine since she was used to eating plants and algae in the pond (plus 1 feeding of pellets per day). By spring there was hardly any of the cambomba plant left. The hornwort was never touched.

This year I am bringing in the goldfish babies that were born this year. I would definetely put in the hornwort (probably get some from my pond). I might still by the cambomba again, as at least this way if I'm worried that I'm not feeding enough, then at least the goldfish can munch on some greens.

LOL, Mary, for the plastic plants. I finally resorted to massive quantities of plastic foliage in the middle of the pond. The little fishies live in it and hide from the bigger fishies who love it for spawning and orgies. Now I have at least a chance of growing some live plants. Algae grows on it so there is a food source for the babies and if you don't look too closely at the bottom you would think they are real. My fish love shelled peas, too. Sandy

Ruby,
Couple comments before you set-up your tank if you haven't already:
With Comets you can go bare bottom with your tank if you want and do more frequent partial water changes if you don't want to do an undergravel filter.
Your will need an external filter such as a Whisper II or comperable for mechanical filtration plus charcoal to clean the water.
If you go with an undergravel filter which is very typical, if your budget can absorb the extra cost I recommend using powerheads instead of airstones. One of the main problems with undergravel is insufficent downflow which allows the detris to go anarobic. Comets are still a type of goldfish and the fish like to graze. You'll find they "pick" at the bottom gravel looking for whatever they can find.
When you initaly fill the tank use your pond water. It is already aged and the proper pH. Do NOT use straight tap water without treating it.
If you don't already have one, get two 5g buckets from Hm Depot or similar. Mark it for aquarium use only. When you do water changes fill one with tap water the day before the change and treat it with a conditioner like Novaqua or similar. Let this set for a day to aclimate temperature and flash of the chlorine typicaly in the tap water. Use the Python syphon to clean the gravel or bottom and drain the water into the second bucket. It's a good way to measure the water you syphon out of the tank so it equals the fresh water you will replace.
If you have water lettus or water hyacynths, toss a few in the new tank. As John said, a couple fish can become more. The fish like the roots of the plants to lay eggs in plus they provide additional filtration.
Add live plants if you can. They help filter the water, add oxygen and just plain look better.

Most of this is under the assumption that you will be setting the tank up where you can watch it and enjoy your fish during the winter. Some people just put a small stock tank in the garage with an air pump and let them go dorment. Tough love....
Have fun with your tank!
JJ

Thanks for the additional responses. I have not purchased the aquarium yet. Actually I was surprised at how expensive they seem to be. Going to look at a local non-chain shop that has lots of fish stuff. I did order a python syphon and an Aqua Clear filter, but they can be returned to a store if need be.

diyguy: Are you saying I don't "need" gravel? What about a hood and light? If I don't need a hood and I put the water hyacinth from the pond in the tank could I just use a free-standing fluorescent light for the plant?

I can give you some more personal opinions of my own to confuse you more if you would like... I would definitely suggest a type of filter other than an undergravel filter. I have used both an undergravel filter and then an external hang on the back power filter (I think I have a Whisper Filter) and the underground filter was o.k for a few tiny tropical fish like guppies but Goldfish are huge chunky fish and make a lot of waste that kept plugging up the filter even with vacuuming with a siphon, plus goldfish love to pick in the gravel and they would keep uncovering the undergravel filter. I think the Aqua Clear filter you ordered is a good choice as long as you bought in a size that is recommended for your size tank (usually the box says what size tank range the filter is designed for so that is pretty easy)

About a cover or a hood, I have covers on my tanks because I am paranoid about fish jumping out. There are the premade ones in the store that have a built in light already but they are kinda pricey, especially if it is sold seperatly from your tank. I am terribly cheap so when I got a used tank from a friend I went to a window store and had them cut plexiglass to fit on the lip of the tank so fish would not hop out. Because I use an external filter there is a little black plastic tube and a "waterfall" lip that returns the water to the tank so the cover stopped about an inch or so from the end to allow for those things to fit in the tank with the lid on. Of course store bought hoods are generally precut to allow for things like filters and heaters to go in.

I guess the question about gravel depends on what type of filter you use. If you use an undergravel filter: yes use gravel. If you use a different external filter like your Aqua Clear that hangs on the back of the tank then you can have a choice depending on how much work and the decorative value of the tank. If you just want the tank to keep the fish alive over the winter then I suppose you would be fine w/o gravel but there would be nothing to hide the fish poop and you would need to siphon rather often to get rid of the poop. Normally gravel is supposed to have good bacteria that helps regulate nitrogen and ammonia levels in the tank and to break down wastes. Plus I just think tanks with gravel look nicer but if this is just gong to be a tank you use over the winter then gravel could be optional, I am sure other people have different opinions though.

I don't know about how well hyacinth would overwinter in a tank...I tried that last year and nothing lived. I am trying water lettuce this year and they seem to be doing ok so far, maybe they are ok w/ lower light levels? The problem I had was since all of my tanks had lids the water hyacinth were too tall to fit in them and have the lids on to keep the fish in so I had to put the hyacinths in a separate bucket.

You probably don't *need* a light if you are just keeping fish in a tank and they are not stuck away in the complete dark but if you are going to use it to overwinter plants too then you would probably need a light. I have a separate light with a reflector shield that I got at a garage sale (cheap me again) and I sit it on top of the plexiglass cover and that gives enough light for the aquarium plants I have (java moss, java fern, some water lettuce, Aponogeton bulbs and Azolla) to grow well w/o too much algae growth. If you know the info on the type of bulb in your light maybe compare it to information on lights they sell for fishtanks? I would think your light would work perfectly well but I just experiment to see what works so I do not have much expertise w/ lights.

Lots of people have fishtanks w/o live plants so seeing as setting up a fish tank with all of these fun toys can be very expensive you may be able to get by with just the tank, your filter, siphon and buckets for cleaning, a bottle of dechlorinizer (cheap), a cover would be nice, gravel is nice too and maybe some plants (plastic or real) to make the fish feel comfortable. They were used to being in a pond below ground but suddenly they are in a tank with see through walls and normally they are a little nervous and jumpy before they get used to seeing all the movements outside of the glass.

Well, this seems to have become a huge ramble but hopefully this helped (?) and didn't just confuse you and make you want to run away screaming.

Just 2 cents - while they are expensive, try to get the biggest one you can afford. The bigger it is, the easier it is to maintain. Really. Take from someone who made all of the mistakes. Several times. Check Craig's list or your local newspaper - there's usually someone moving and selling the aquarium.

I'm incredibally CHEAP. I see full aquarium set ups very cheap at yard sales, flea markets and you could actually go to freecycle.com, find your area and ask for a set up from there. I see them quite often. I guess craigslist also has a free section.
Worth checking out!
NT

Thanks again for more responses. But now I have a new dilemma! I went to a local privately owned pet equipment supplier that has a large fish department. I did see some rather inexpensive tanks. But when the fish Guru came to help me I told him that I wanted a tank to overwinter my pond goldfish. He asked me "why?" I said that my pond will freeze and when I told him the dimensions 3 x 5, mostly 20" deep but with dug-in shelves on 3 sides and one small area 24" deep, he said that with a pond heater I could still overwinter the fish in the pond. He said that it wouldn't freeze all the way down. We live in the Chicago area so I am fearful of doing this, but this guy is local also.

What do you guys think about this? I am going to search through some posts that may deal with this subject.

BTW, Petsmart.com cancelled my Aqua Clear filter order (I guess they don't have it yet) and the syphon thing is on back order.

I have started a new thread to deal with the question of leaving the fish in the pond for the winter. But I am still interested in the aquarium help.

Thank you all so much for the replies.