Grunch

This is an automatic bet, villain showed weakness by checking, you have to fire and try to take the pot down right here right now. $8-$9 would be a good bet size, with a half-pot sized bet, now you don't know where you stand when villain raises.

Checking behind is generally a bad idea in situations like these because if you don't improve on the turn, what will you do when your opponents makes a pot sized bet into you? You have no idea where you stand at this point and you're either forced to lay down your hand or fire back at him, which you are now paying a lot more for essentially the same information.

With no reads on the villain and given that this is NL25, I doubt villain has you beat on the flop. He's most likely semi-bluffing with a flush draw, possibly KQdd or KJdd.