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Hello everybody, today I was thinking of filling my turtles tank up with more water but he grew up in water where he just sticks his head up out of the water i had him for 6 years hes in a 40 gallon tank he eats pellets and lettuce so my question is should i fill his tank up or not cause I dont want my turtle to drown also what kind of water filters do you guys recommend? thanks in advance!!

How big is he? As long as he can't escape and can easily access his basking area, he should be fine.

As Steve said , yes. You also don't want any tight places he /she could get caught in , make it's home safe. Watch out with caves they sometimes try to turn around in and get caught. I always make something like that twice as big as turtle for safety. Aquatic turtles are great swimmers and love it. That is juveniles and adults.
If small still could add water just offer resting places if it got tired. Floating plants...

He is about 6 or 7 inches grew up in like 4 inches of water

If a 40 gallon long it's 48x13x17 ...
To me shallow water can be a hazard to RES's if they fall on there backs not being able to correct himself and drown ! RES's are aquatic turtles that are great swimmers and love to do it ! This is old school but water depth for him should be 2 - 2.5 times his shell length. That's 12" to 15" of water and tank is 17" high. How about an "ATBA" and give him a full tank he will love you for it ,that's if you can swing it or DIY. Make sure it's escape proof ! Really looking at an upgrade for 6" turtle... 10 gallons water for every inch of shell , that 60 gallons water alone ! That's why I use Rubbermaid stock tank's cheaper compared to glass tanks.

thanks everyone for the advice