Taking in a turtle is not easy , must do a lot of homework ! Not just reading a basic care sheet. Each stage of life it changes and they get big needing a large habitat. That's not just for more swimming space but to control the waters parameters. Most turtles are given up to new beginners because they are not the easiest or cheapest pets to start with on a proper initial setup. Sorry but from your photos the water is way off. Going on six decades in this hobby most old long timers can tell by looking. What are your nitrate number First ?
I used my software to see worms better on my PC with a 5D monitor. You actually have a few different type’s in photos. Detritus and planaria are the most. Yes what's small to you is about the third stage of life but been there a good while. They start out microscopic. Comes as eggs in live plant’s , contaminated food mostly. Then thrive in less than clean water. Also when things are not cleaned well and quarantined they enter the tank as hitchhikers.
Filters are rated for “fish” gallons and comes with media for fish ! Yours yes is rated for 200 fish gallons but a turtle requires 2-5x’s the flow of fish being how much bio load they produce. Like 2x’s for baby turtle and up for larger sizes.
The bubbles from mouth and nose is a bigger problem. Second stage of “IR” disease. What you have done in the beginning is not what it requires as it grows and or what previous owner said , needs changes on habitat . Please read up whats needed for your species and offer more time and attention to habitat.
You now need to break down all again and sterilize tank , every rock , decorations get rid of filter media and get new ! Only way to correct and get all stages of life on them , microscopic and up. Work on cycling your tank water ( nitrogen cycle) and maintain it cycled , get rid of fish media and use a turtle media as in bio. That filter is good for a turtle up to 2 1/2” only to be adequate with good turtle media. If you keep it at least set it up with a turtle media setup and use a nitrogen cycle to help an inadequate filter for your turtle. An adequate filter the day before cleaning the tank water will be clean. That's because an adequate filter and good media setup will catch and hold waste not send it back to the tank.
You must read up on respiratory infections and dry dock along with RX meds to treat your turtle , “IR” infections are fatal if not treated. No OTC meds will help with "IR" !!! Then have tank setup better for your friend without any hitchhikers in it.
Sorry but more time and attention is needed for your turtles needs , I am here for the turtles. Correct and treat the “IR” before the infection get’s into the blood stream which is fatal.
If too much for you seek out a rescue center , SPCA , or maybe a nature center for it's home.
Last edited by litefoot; 06-20-2019 at 08:48 AM.