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

A small fish tank is any aquarium under ten gallons (about 39liters) total capacity.
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A little research says that platies grow from 1.5 to 2.5 inches, depending on gender. I definitely wouldn’t add any other fish to your tank, since it is so small, but if you watch the water quality carefully, you might be ok. It sounds like platies are pretty hardy, which should help. Of course, that doesn’t mean that they should be in bad conditions just because they are hardy.
Small Fish Tank Decoration Ideas ~ Garden Image Decoration Design Idea
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Nearly 13 million American households contain a fish tank, and the average tank size is less than 10 gallons. Yet comparing the behavior of common freshwater fish in a variety of habitats found that those kept in such small tanks were considerably more aggressive than those in larger ones — more likely to fight, flare their gills and guard whatever tiny alcoves they could find. Also, if you are going to be running a small fish tank setup be prepared to perform  more often.
Photo provided by FlickrBought, a fighter fish to my collection of small fish tank.
Photo provided by FlickrSmall Fish Tank Decoration Ideas ~ Garden Image Decoration Design Idea
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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.If your fish tank is either under ten gallons in totalcapacity or is not filtered, it is a small tank or bowl andshould only be considered by experienced aquarium keepers whounderstand the drawbacks and disadvantages of such an environment forpet fish, and are willing to take on the extra challenge and additionalmaintenance required for these small aquariums.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.Because of their small size and frequent lack of filtration, smallfish tanks and fish bowls require a great deal more maintenance than alarger fish tank that has a reasonable probability of becoming - andremaining - stable.This is a really hlepful article about setting up a small fish tank. The person actually knows both about caring for fish, and setting up an attractive looking aquarium. Spent 20 minutes looking through my internet history to find again. Pinning, so I don't lose it!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.