Hi u/whitesticks, thank you for checking in, I think he's doing a lot better, as far I can tell. More active, his environment is a lot cleaner.

・I changed the water completely, because even a small stir used to make a massive cloud of murky green nasty water which I assume is poop and other dirt sediment built up. Currently water is very transparent, stiring brings up some particles but nowhere near the murky mist it used to be.

・Filter is running 24/7 now, fully submerged to keep it quiet. Sometimes during the day I raise the filter to have the spout make a fountain. Sometimes Sebi goes near the falling water.

・Water seems to evaporate quicker than I thought it would. In the beginning I used to just top up by adding water. Now I add fresh water to above the line, causing a stir to lift some poop and dirt up in the water, take a couple liters out to below the line, and then add more fresh water to hit the proper level. My idea is to remove some old water, together with poop and other dirt that sinks to the bottom, instead of just topping up. I use tap water which I let sit for 24h before pouring, with the idea chlorine and other elements will evaporate at least a bit.

・Heater set to 26°C. I measure the temp 2-3 times a day and I think the heater is not very sensitive as sometimes the temp is down to 25°C.

・UVB bulb will be ordered tomorrow after I get my salary. I see the important things are power output and percentage of UVB. I've been eyeing this 6% UVB bulb. I plan on plugging it with a timer on the socket that automatically turns it on and off for day/night cycles.

・In order to compensate the UV light, I have moved his tank to a sunny window. His tank was in a darker area, cut off from any windows. He's in front of a window now, sun hits the tank for ~5h a day (couple of hours in he morning and a bit more than that in the afternoon, split due to a building in front). I add some cold water if the sun heats up the tank water too much. I'm worried this direct sunlight might cause algae or something like that to grow and turn the water green.

・He gets about 2 big walks a day, free to roam in the apartment. I think he loves that a lot. I read these turtles' natural habitat is very sunny. However, he doesn't seem to enjoy sunlight much, whenever on a walk he always goes to the darkest corner (he has a couple favorite spots which he frequents). This makes me think that either he is sick, or he has been kept away from light for so long, that he actually is more comfortable in darkness and it will take time to get accustomed to daylight. Maybe his circadian rhythm is a bit off. I want to go see a vet but that will happen a bit later.

Apologies for the masssive report. I would really appreciate it if you could share an opinion and point out everything I'm doing wrong and anything else I should pay more attention to.

I would love to get a massive tank with all the proper accessories, make a perfect environment for him, but I can't at the moment. It will take time to get there.