Hi!
Do you have a water testing kit at all?
I am suspecting that the nitrate and ammonia levels in the tank may have peaked especially as you have had him for exactly a month. Whenever you set a tank up for fish or turtles there is an unavoidable peak at around the 30 day period. Often this catches new owners out as they lose all of their fish as they struggle to live in the extreme parameters. Turtles are normally a bit tougher. But for them ammonia is a real issue.
It is advisable really to set a tank up and leave it livestock free for at least 30 days before adding fish or turtles

You can buy good bacteria to set up a tank and these are also a good idea, but I wouldn't rely on them. You can also use a bit of already 'levelled' out water from another tank which is known to be disease free to kick off a new tank's good bacteria
If you don't already have a water testing kit - please do get ASAP. You could use the paper strip type to test in this instance as quick, but the bottle/test tube type are better really. Just the basic set to test for PH, nitrates and ammonia.
In this first instance I would recommend you do an immediate water change. Say 20% out and a new 20% in conditioning the water with your water conditioner even 25%. Make sure you match the temperature well too.
You can also buy an instant ammonia neutraliser from the fish/turtle shop. You just add per instructions on the bottle.
The parameters will start to fall immediately with the water changes and if necessary repeat every day for a few days. Your little fella should start to be a lot brighter.
Also make sure that your heater is working and set to the correct temperature (normally around 27 deg C). Also that your filter is working well and efficiently as turtles create a lot of waste.
I am reasonably sure it's the above - my apologies if I've doubled up on others posts elsewhere, but I'm not sure which posts you are referring too
Good luck!

Let us know how you get on!
