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I have been asked to take two turtles from a family member. They are each 5 years old, one a red eared slider about 5 inches long, and the other a western painted turtle about 6 inches long. They are currently both being housed in a filthy 5 gallon aquarium with about 3 inches of water. All I have on hand is a 20L (with filter, UVB lights, etc). While I know this isn't anywhere near large enough, would it be able to support them short term? Also, they currently have no basking area or lights. I have the lights, but I need a cheap(and quick) idea for a basking area for them that would allow as much of the tank for swimming room as possible. I do plan on upgrading to a much larger enclosure in the future.

20 is definitely a upgrade. Poor babies probably have horrible pyramiding. You need to buy a at least a 60 gallon asap. But cheapest way to build a basking area? Is real rocks. Steal some from a park or garden, clean them and stack them. You can google some DIY stuff out of pvc piping and wood. If you can afford to spend up to $40 you can buy one. Penn plex in large are great. I love my corner one. There's also a filter/basking dock one that's cute and looks like rocks.

Good for you for rescuing them from a horrible situation!

I assume you mean 20 gallon not 20 liter. Even that's not really big enough for one of those turtles, but is definitely an improvement.

An above tank basking area is the way to maximize water space. But, really you need something much bigger. You might want to look at stock tanks, Rubbermaid containers designed for watering cows or other stock but often used by turtle keepers as economical alternatives to glass aquariums.

Do you know their genders? The painted is likely a female at that size.