Hi Elka, here's a how to! https://www.petmd.com/reptile/care/evr_rp_turtles-101-how-to-clean-and-care-for-your-turtles-tank
Turtles are ALOT of work.....some turtles are much more care than others.Buying a good filter for it is a must therer are ones especialy for turtles, remove any feces immediately and buy bio chemicals that help break down waste and establish environment. Turtle tank owners with aquatic turtles, though, know that there’s an extra level of challenge when it comes to giving these friendly reptiles a good home. You’ve got to make sure that tank is big, you’ve got to change the water frequently, you have to keep the water moving around, clean up waste, monitor chemical levels, and you need a good canister filter to get all of that accomplished. The water can become stagnant if it is not moved around, which is a surefire way to decrease the quality of the water increase chance of disease. Circulating the water helps to evenly distribute oxygen and good bacteria around the tank, discourage the growth of harmful bacteria, keep water temperature regular, and slow down algae growth.
To make a long story short, a weak filter is not going to be able to handle the sheer volume of water you have and provide all of those essential benefits, so if you’ve got turtles in your tank, you have to go big.
With smaller water-dwelling creatures, you can sometimes get away with a slightly underpowered filter to save yourself on money or to avoid having to carry the thing around. With turtles though this option is foolhardy. Power is a non-negotiable component of a turtle tank filter.
There are other factors you’ll need to have aside from pumping power, though. A powerful filter will do you no good if it can’t hold enough of the right type of filter media. You need to make sure that whatever filter you decide to go with has a large central chamber or multiple large trays to accommodate a lot of filtration.
You’ll also need to make sure that you can customize which kinds of media you use in the filter, as your turtles will need a very particular water chemistry to stay healthy, and you have to be able to fine tune your parameters to achieve those levels. https://www.myturtlecam.com/water.php more https://theturtleroom.com/blog/2012/11/16/the-complete-balanced-filtration-system/ more https://www.myturtlecam.com/turtle-tank-water-cycling.php
Ilka: maybe this will help
Every day: Use a small net to remove any droppings or uneaten food. Every couple of days: Test the pH level of the swimming water. It should be between 7 and 8 on a scale of 1 - 14. Get a test kit, or bring a water sample to Pet Smart. They will test it for free. At least once a week: Replace some of the water in the tank with clean water. Every 2 or 3 weeks: Clean the whole tank and refresh the filter.