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My turtle used to poop once a day, but recently about since 2 weeks ago, he poops about every 3~4 days, that really worries me, I didn't change his diet pattern or anything, and he basks and swim, the water is around 78 all the time. Is it possible that he ate it? is it going to harm him if he does? do you think feeding veggies would help?
(but my turtle used to never eat veggie and never had problem like this...)

If he's eating food, he's pooping, and he very well could be eating that as well. If he's healthy, it shouldn't hurt him. Turtles do eat their poop with no ill effects. Offering some veggies, especially leafy greens like red leaf or romaine, lettuce, dandelions etc. is always good (take a look at the feeding list).

If he were constipated, he'd be kicking at his but with his hind legs. Is he doing that at all?

i saw him kicking his butt before, but very rarely, I just wish I could have my eye on him often but since I had to go to class

If you really think he's constipated, you could put him in a container of water that's a few degrees higher than normal (not too warm, though). You could feed him in it and then leave him in the water for a while. The warmth relaxes the internal organs and makes it easier to poop (and some turtles poop shortly after they've eaten). I once had a turtle that had trouble pooping sometimes, and that's what I did. If you're not seeing any butt kicking or it's very rare, he could just be eating it, though.