my older sister and her boyfriend moved to his late grandma's house, and she (the grandma) left this turtle to them. I don't know anything at all about water turtles, but i had a tortoise for a couple of years. no one knows the age or the species of this turtle. we just know (or at least, i was told by my sister's bf) that it's a girl).The water is changed daily, but no antibacterial products are added (i know that people add them to fish tanks). there's also no "soil" (like bedding) and no elevated space (like a rock to rest outside the water). it has been fed turtle food; apparently, the grandma fed her "when she remembered", now my sister feeds her once a day. it's late at night rn so i can't do much, i've just petter her lightly. she's a bit "slimy" (like similar to marimo moss on top of rocks in the sea), so i was thinking of brushing her gently with a toothbrush tomorrow (my tortoise loved it, so i was hoping it's the same for water turtles. let me know if it's wrong to try to "clean" her like this. I won't add any chemicals or soaps, since i don't think it's a good idea). i wasn't able to measure her or weight her, unfortunately. maybe tomorrow i can. also i'm sorry for the shitty pics.

i'm a teenager and english isn't my first language. also, please don't get angry at me. i'm trying my best, and i just became aware of this poor creature today.
 

turtle neglected for possibly years. please help with advice and info about anything you can think of.