In our opinion the best filter for a small fish tank would be the

Lemonade/drink dispenser for small fish -drain 2/3 to clean tank & use old water for plants!
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works very similar to the sponge filter, they are powered by . So these filters don’t stir up the water too strong that makes them the best filters for Betta fish tank as well as for other small fish that don’t like to live in the turbulent water.
Oxygen isn't really the concern. It's waste. Too many fish in too small of a body of water create too much pollution for the tank to handle.
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Remember that even if you take these precautions, and make aneffort to make the small aquarium or fish bowl a better place for yourfish, you still have not overcome that small water volume issue, whichis the primary problem with these fish tanks. Adding a filter oragitating the water will not significantly reduce the maintenancethese small aquariums require. This is also an ideal filter for small tank with Betta or other small fish. The filter is loved by its powerful features such as s
Photo provided by FlickrSome fish that are recommended for the smaller tank are mollies, platies, tetras, guppies, danios etc.
Photo provided by FlickrI like Tetras they stay small and you can find fish with really nice colors. Plant live plants and the tank will be very easy to care for.
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Finding small saltwater fish tank species for a small marine aquarium (nano tanks) is not difficult and they are often easily found in local saltwater reef stores and online. If you have a small saltwater aquarium it is vital to keep it lightly stocked and your water parameters stable. Make sure you research any fish you decide to get before you go to the store. Do not make that impulse buy or you may regret it later.However, many do start with just a small fish tank, so we want to give you a list of items to use with your small aquarium so that you can increase your chances for success in tropical fishkeeping. If you need help with setting up your new tank, check out the page.Any fish tank from 10 to 30 gallons could be considered a small saltwater fish tank for the purposes of this article. Really, any saltwater tank under 30 gallons is going to be a chore to maintain (my opinion of course, to each his own). Nano tanks are often considered even smaller. It should also be noted that the smaller the fish tank the harder it is to keep in my opinion. Larger tanks provide so many more options for both equipment and fish species and I strongly urge any new hobbyists to get the largest tanks they can accommodate. Leave the smaller specialty for when you become more experienced later on.Female guppies grow to 2 inches and the males are a bit smaller at around 1.5 inches. Their depending on the genes, food, nutrition and more. They are known to be small fish and get pushed around in community tanks for that reason. Be careful of larger fish trying to eat guppies. Guppies are very small fish but pack a tremendous appearance.Below is a typical start-up kit for a small 2 gallon mini-bow aquarium. It comes with a lighted hood, a small packet of sample fish food, an undergravel filter and a small air pump. You will need to get some other items as well for your aquarium. As mentioned previously, a small tank can be more work than a large tank because you really have to stay on top of those water changes to prevent the small aquarium from becoming too polluted. Here is a list of the bare minimum things to have for any small aquarium:These small beautiful fish can be kept in community tanks. Just make sure these tanks are tropical fish with heated water. You will need an for these setups. Check our top 20 questions posted below. Submit any questions you have for new threads here.