since few days its been eating as usual but he is not pooping the water is clean.. before he always used to poop everytime i fed him what is wrong?? is he sick? can he be constipated??

It is possible that you aren't feeding him enough and he is eating the poop, or it is a little softer than usual so the filter is dealing with it faster. Otherwise, if the temperature has dropped and he doesn't have a heater he may be planning to go into hibernation. If you get really worried, bathe him in warm water (probably lukewarm, to your touch), and that should get him to go.
Has he been flipped or turned sharply and/or suddenly recently? Turtles insides can get tangled.

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Good luck. Hope this helps.

Experience and all the times I've panicked about my RES

I'll bet you run for a laxative if you don't poop every day. Relax. The turtle will poop when it's ready to do so. If you have to do something, feed it some liver. That will help.