Cleaning the tank
When cleaning the tank, I would recommend that you have:
Magnetic Glass cleaner
Gravel siphon
Filter (generally used in the tank but needs to be cleaned out regularly)
While the filter can do a great job of keeping the tank clean, you should ensure that it is washed out regularly and that you use a gravel siphon every week or two to keep the tank clean (the smaller the tank, the more often it will need to be cleaned out).
Never remove all of the tank’s water, try to leave ½-2/3 of the water in the tank and replace the water you have removed with tap water, including water conditioner if you have fish in the tank. Also, as the glass can become quite dirty, we generally try to use the magnetic glass cleaner before siphoning the gravel (in the hopes that some of the muck coming off the glass will be sucked up by the siphon…).
Checking the shell
To check the shell, pick up the turtle holding it with your thumb underneath and fingers on top and gently push the shell to check that it is hard (do not push too hard as they can feel through their shells and do not push on baby turtles shells as they are soft and will harden as they get older).
Monitoring temperature
If you have a water heater with a temperature setting on it and put a stick on thermometer at the other end of the tank (we use a flat stick on one which sticks to the outside of tank, it gives a reasonable reading and isn’t in the tank to bother the turtles).
It is a good idea to put the water heater close to the filter, so that the heated up water is pushed around the tank. By just glancing at this thermometer every now and then you can ensure that your heater is working properly and see if there are any adjustments that need to be made.
Replacing plants
Check plants regularly and replace them as need by (generally plants will need replacing every few months as they are eaten/worn away).