another link
http://www.petfish.net/kb/entry/263/

A two gallon tank would be fine, if maintained properly, for a pair of guppies, a pair of mosquito fish, or any number of small killifishes, such as the Lyretail Killie (Aphyosemion australe) or Clown Killie (Epiplatys annulatus). By maintained properly, I mean that you would have to do partial water changes at least once per week, maybe twice.

Joel

In my opinion and experience, the Paradise fish is a better choice for the small tank and will thrive in your two gallon tank if properly set up and maintained. Here's an article I wrote on keeping them happy and healthy in a small planted tank. You can find Paradise fish in aquariums stores in the US, Europe, and even the Middle East.

The Low Budget Natural Fishbowl

It does not take much time, money, or skill to create an affordable and beautiful, ecological, low maintenance aquatic world you can enjoy for years. While some fish traditionally sold for fish bowls do not thrive in them, there is one kind that most definitely does, macropodus opercularis, or, the Paradise fish, one of the first fish ever kept in home aquariums. Bettas, often sold in tiny bowls or vases with plant bulbs, require warm temperatures and do better in heated tanks. Goldfish, also frequently sent home with bowls, require large amounts water to thrive. The ones in stores are tiny babies these fish are meant to grow quite large and need a tank of 30 gallons, each, to do well. Paradise fish are a much better choice for a natural fish bowl or small tank. They are extremely hardy and long lived and they do not require any special care or conditions. Like Bettas, they can breathe air through a special labyrinth organ and so do not require an aerated tank. They prefer cooler water and so do not require a heater. They also do well living singly. Males, like male Bettas, will fight if kept together in close quarters. Mostly males, because they are more colorful that the females, with bright bands of turquoise and orange, are what you are likely to find in the pet store.

The fish keeperÂs goal is not to have fish that merely survive, but rather that thrive. Here is how to provide a healthy home for an easy to keep, colorful, and intelligent little fish. These recommendations are based on my own set ups that have done well for years. You do not need to have ever owned a fish before to succeed with a Paradise fish if you start with a young, healthy fish (look for a small one, under two inches from tip to end of tail and ask to see it fed) and follow these recommendations.

Shopping List:
Large fishbowl or glass container (preferably 2 gallons or more in capacity)
Small amount of bagged potting soil
Small bag of aquarium gravel (natural, smaller sized, dark in color)
One or two live aquarium plants that do well in cool water -- a bunch of Elodea or a Java fern will work; get small plants as they will grow quickly in this set up
Spring water or tap water aged for three days and treated with water conditioners to remove chlorine and other chemicals that can kill fish. In the US, I found it easiest to just keep several gallons of spring water on hand. Also, in case of emergency, a gallon container of spring water with the top cut away makes a readymade holding tank for a fish.
1 Paradise fish
Small canister of tropical fish food flakes
Optional:
Small net to catch fish
Frozen food and/or freeze dried bloodworms to vary diet
Decoration  keep a natural look with a stone or two (to be safe, use those sold in pet stores), or a very small piece of driftwood from the aquarium store, pre-soaked for a week or two in a bucket of water to remove tannins and water log it so it won't float, will look fabulous, especially if you attach the java fern to it by tying it with a bit of dark thread. Soon the fern will attach itself to the wood and you can remove the thread.

Set up:
1. Clean bowl with salt and warm water, rinse well, and dry. Never use soap!
2. Line bottom with an inch of potting soil.
3. Remove plants from metal bands or pots and place on one side of bowl to create a backdrop effect and leave swimming room in front. Tall plants in back, smaller ones in front if you have more than one.
4. Add a one inch layer of gravel over soil and around base of plants.
5. Place a bowl or cup upside down on gravel and pour bottled water slowly onto it to fill bowl without disturbing the substrate. Fill about 4/5 of the way, stopping at the wide part of the bowl because more surface space means more oxygen for the fish. Pour very, very slowly so the gravel is not displaced, allowing the potting soil to mix with the open water. Even if you are very careful, the water will still be a little murky when you are finished, but this will settle in a day or two to crystal clear.
6. Add fish, following pet store instructions for acclimating a fish to a new tank (float bag, add small amounts of tank water to bag periodically, etc.)
7. Place bowl near sunny window for natural light to maximize plant growth

Maintenance:
Every day, remove one cup of water and replace with one cup of fresh water (bottled or treated) and top off evaporated water as well. This takes less than one minute and assures a healthy environment for both the fish and the plant. You do not have to do large water changes if you keep up with this every day. Do not just top off, however. Removing some of the old water will remove toxic nitrates and help keep the water clean. You donÂt want to remove too much, however, because the it contains also contains beneficial nutrients and bacteria.

Some people feed their fish every day and others prefer to feed every other day. I feed mine as mauch as they can eat in 20 seconds, usually a couple of large flakes, a couple times a day. With the daily one cup water changes, this works well. Be forewarned: if food is sinking to the bottom, uneaten, you are overfeeding. The fish can easily miss a day or two without a problem -- underfed is actually healthier for the fish than overfed -- but if you will be away for longer that a few days, find someone to care for your fish in your absence. Better not to leave the entire canister of food with the caretaker, however. Just a pinch in a baggie is enough. This way, your fish is less likely to be overfed by accident. Overfeeding will pollute the water and undo all your effort to create a healthy natural environment for your fish.

The plants will be well fed first by the nutrients in the soil and then by the fish waste. You will not need to fertilize them.

Trim plants as necessary. They will sometimes grow very quickly in this sort of set up. My elodea was growing new shoots within days of being planted. Elodea can be broken off when it gets too long and the new section can be planted, thrown away, or given to a friend with a tank or bowl. To plant a new shoot, just remove the few bottom leaves and insert it into gravel and soil. Java Fern will grow new plants on runners. These can be left alone or plucked off and planted or given away.

If algae should accumulate, it can be wiped off with a paper towel. The plants and water changes will likely prevent algae from forming. I have never seen algae in my small tanks, even though they receive a great deal of direct sunlight.

Please note that with this set up, you would be wise *not* to vacuum the gravel or to dump out entire contents of bowl. The fish waste is necessary to the live plant and works with the potting soil to provide essential nutrients. The plant, in turn, helps keep the water healthy for the fish. The small daily water changes will more than suffice for keeping the water healthy.

ThatÂs it! Enjoy! I have kept this sort of natural fishbowl in three different countries, on three different continents and have set up Paradise fish bowls for countless friends who have admired mine. Paradise fish are easy to find and easy to care for. They make delightful little pets and will charm you with their antics and desire to interact with you. They will soon recognize you and will feed from your hand even. The fish and plants absolutely thrive for many years. You can give your fish a larger home later if you like, but it is not required to maintain a healthy fish for years. My first Paradise fish, now age 8, is still going strong and the plants must be continually pruned so he has swimming room.

One note of caution: Paradise fish do like to jump. I have never had one of mine land outside of the bowl but I have heard of that happening and mine have surprised me by jumping impatiently to take the food from my hand before I could let go of it. It is wise to cover the bowl with a bit of netting or a lace doily (something that lets air in and keeps fish in) and a rubber band to prevent accidents.

Bob - I know from keeping bettas (mostly rescue) that some do better in small tanks rather the large (maybe long fins flapping around and gets damaged and fin rot sets in. I tried 20 gallon, got sick, but did fine back in 2.5 now in five planted tank doing fine). (The cells of the tails of these hybred types are one layer thin so easily damaged unlike their wild counter parts.)

I would say one mystery snail but betta may stress him with nipping on feelers, but that's about it. Would eat any shrimp or chase smaller fish and stress would kill them.

If you got a larger size of at least a ten gallon, I have kept priscella tetra school and black neon school (not together)(about 8-10).. Looked good together and did fine - they were in with a female so male MAY be too agressive.

But definately not for 2 gallon. Guppies with bettas for instance would have their fins nipped. A larger tank opens up all sorts of possibilities, although I have had best luck with small schools as the betta gives up on chasing 'the larger fish'...

I have one really pretty long finned male in large round globe (about 15 gallons of water) with wood piece planted with Anubias... One mystery snail, spot lit, really pretty to look at. (I had small school of neons (black) but unlike when they were in with a female, this male did not leave them alone so I moved them back into larger 30. He will remain alone with his mystery snail companion. (he can see another male in tank nearby - ten gallon)

It is not also the space that each needs to feel like it can roam and explore and exercise (my bettas really go to town in 5 gallons planted and with wood for variation) but the bio load and the larger the tank size the more stable the water chemistry. Guppies are fine but they also like to swim around and you could only have males as the ratio of male female makes this size inpractable... several females for one male or they get chased to death.. and then there are those guppy fry that will soon over stock this size. So a few beautifl males might do ok. 5 gallon would be better though. I agree with other ideas on shrimp and smallish fish that stay small.

Best Sherry

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Neshenny: (first off I don't mean anything offensive in my responce to you)

You don't mean this Paradise fish do you? I've kept this fish and can't imagine what you are suggesting.

If so then whole heartedly disagree with your opinion. They like all fish including Betta's do not thrive in tiny space just as they do not in the wild.

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(note the minimum tank size - min is 20 but most keepers keep them in much much larger tanks...

Note the 'very active'... well guess not in small bowl, survive maybe as they are Labyrinth fish like Bettas doesn't mean they should be equally abused mho (and theirs I am sure, lol).

Nesheny - please rethink your position if we are talking about the same fish.

(ps.. .none of these in the wild are in small puddles - another myth pet stores spread to assuage guilt when they sell bettas in inappropriate conditions - rice paddies are huge systems much like a swamp system, slow moving but nutrients in, waste out...)

http://en.wikipedia.org/wiki/Paradise_fish

"In Home Aquarium

Male paradise fish should be kept apart, since they will fight aggressively by locking jaws. Male can be kept with females; females may also be kept together in groups.
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A tank that includes paradise fish should be at least 20-30 gallons in size.
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The tank should be well planted and covered; bogwood and rockwork may be included. Paradise Fish are often aggressive thus tankmates must be chosen with care. Suitable tankmates include giant danios,large tetras,most smaller catfishes and even some of the less aggressive cichlids. Slow moving or long finned fish such as fancy goldfish and angelfish are likely to be attacked, bettas and gouramis may also be victimized due to their resemblance to paradise fish. Small fish such as guppies,neon tetras and zebra danios are likely to be consumed."

http://www.aquahobby.com/gallery/e_Paradise_Fish_Macropodus_opercularis.php

Here is an excerpt.

"This fish is a denizen of slow moving or still bodies of water in the wild, so that's what it should get in a aquarium. slightly turbulent water in my experience, lowers male aggression by preventing bubble nests from popping up in the corner of the tank. These curious fish should have some rocks to explore, some plants to drift through, and maybe if you are want to create a even more natural habitat and fork over a little more money, some driftwood for them to look at.

These fish are very active, and have a habit of pacing back and forth through the aquarium (inspecting everything that is new).

They also like to glide through the plants, inspecting all possible morsels of food. If a new fish is added, they may huddle around to view it, and maybe pick at it (and that will scare the living out of a shy fish).

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They should have at least a 20 gallon tank.

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"Living in smaller tanks is not very fun for these fish, as they just sit listlessly in them and do nothing. These are not like the bettas, they like to move about.

Plus they are much bigger than bettas. This is not saying bettas don't need room however. "

Well, Sherry, I've kept them for years, successfully. And I have yet to find that a single one needs 20 gallons or slightly turbulent water. Now, if, you want more than one or you wish to keep them with other species in a community tank, they YES, you need space. They are aggressive when they reach maturity and need space to lessen aggression. You can keep multiple females together quite readily, but they are not easy to come by in most LFS's. Have you ever had one by any chance? By all means, if not, and you choose to own one, put it in 20 gallons if you wish, but for the single paradise, that's a bit much. The advice I've offered is based on experience and success. I'll let you know when one of my guys becomes listless. Hard to imagine.

Reading about fish is one thing. Owning and raising a particular species is another.

I would be glad to share photos of not very listless but very happy paradise fish in their own small planted tanks to anyone interested.

Links on keeping a (single) paradise fish:

http://www.sicktank.com/
(Scroll to Dr. Erik Johnsons article on Paradise fish)
http://www.answers.com/topic/paradise-fish
http://faq.thekrib.com/fish-popular.html
http://www.angelfire.com/home/fishbowl/fish.html
http://www.fishpondinfo.com/oldsite/fish2.htm

One more link:

http://www.aqualandpetsplus.com/Gourami,%20Paradise.htm