... I do 25% water change weekly, sometimes fortnightly.
Hi
The turtle must have a place to get dry and warm. It is not a fish. You know it will grow 30cm diameter. Also, turtles love raw chicken cut to small portions.
The water changes are critical with nitrogen factories turtles. You will need to monitor NO3 and possibly increase water changes if NO3 is over 20 – 30 ppm. This NO3 is slimy waste that is not fully available to plants without massive biological filtration.
I would add a balanced fertilizer to every water change. No daily dosing. This way you are sure plants are getting everything they need and the turtle has clean environment. For example, if you change 40L then add PPS-Pro solution #1 macros 10 ml and solution #2 micros 1 ml or 20 drops.
Solution #1 macros 1 ml per gallon or 4L
Solution #2 micros 0.1 ml or 2 drops per gallon or 4L
This makes 10 ppm NO3, 1 ppm PO4, 13 ppm K, 1 ppm Mg, 0.1 ppm Fe(TE).
The fertilizer quantity follows the amount of water that is being changed. I think this is the easiest way to maintain your turtle world.
How many hours is your light on? Average is eight hours.
Edit: Maybe better to use
PPS-Pro #1 macros NO3 Free solution instead, as Maryland Guppy suggested. This makes 0 ppm NO3, 1 ppm PO4, 13 ppm K, 1 ppm Mg, 0.1 ppm Fe(TE).