What species of turt is that? Looks like a red eared or a yellow bellied slider. Either way does he have a place where he can completely get out of the water? A heat lamp is crucial as well for these guys, since they cannot regulate their internal body temperature like we can. I have a red-eared slider in a 55 gallon with water about 1/2-3/4 full and a mound of rocks for him to climb up.
Your set-up looks awesome though...and your turt looks like he has a good bit of personality. They really are cool pets.
55 Gallon - Reef
55 Gallon - Red-Eared Slider
20 Gallon - Brackish/Figure 8 Puffer