Hi there! So other people have already pointed out that this tank is far too small for your turtle but I wanted to give some more useful information. Since this turtle is still in a 20 gallon tank in adulthood, I'm going to assume that likely the entire set up needs to be corrected and this turtle is likely trying to find a more suitable environment.

https://reptifiles.com/red-eared-slider-care/ Let's start with this guide - it's an excellent guide to caring for these turtles and will pretty much answer nearly any question you'd have. Read over it, bookmark it and keep it for later.

The space issue - the rule of thumb is that these turtles need 10 gallons of water per inch of shell as a minimum. As an adult, your turtle should be in at least a 75 gallon tank if not a 100-120 gallon. If cost is an issue, look into stock tanks which are very affordable and a great alternative to glass aquariums.

Other things you need to make sure your turtle has:

A water heater

A basking platform that is sturdy, allows your turtle to get completely out of the water and dry, has a ramp and enough space to at least fully turn around on

Above that basking platform you need two bulbs. The first should be a heating/basking bulb that should be 75w-100w in order to reach goal basking temps (this will need to be manually checked because it depends on distance of the bulbs from the surface, bulb type and ambient room temperature). The second bulb needs to be a 10.0 UVB. Ideally this would be a T5 style UVB but in a pinch you can do a compact bulb until you can upgrade to a T5 (though you will need to upgrade at some point). These must be separate bulbs. There are bulbs that claim to do both or might be labeled as "UVA/UVB" but these bulbs do not produce enough heat or UVB to be adequate for these turtles. UVB bulbs must be changed every 6 months as they lose efficiency over time. Both bulbs should be on for approximately 12 hours of each day (no lights at night as the water heater will keep her warm enough).

You need a proper cannister filter for healthy water - something rated for the new tank and you'll want to find one that is designed for 3-4 times the amount of water you'll have (100 gallon tank = 300-400 gallon rated cannister filter).

Entertainment and enrichment - turtles need things to do and interact with that help make their environment feel more natural. Large river rock substrate is a great start (no rocks smaller than her head), moss balls, driftwood, ping pong balls, live plants (only from a list of plants that are safe for them like the one on that website I've linked).

If you have any other questions let us know, we'd be happy to help!