If the cloudiness seems to have a green tint, it is probably an algae bloom, like user up there said! My suggestion is leaving the tank lights off for a day or two, covering the tank to provide minimal light (even room light preferrably), and it should be gone within a couple days. Any plants in there shouldn't suffer too much from this small period; also make sure to not feed (or feed minimally) for the period. Always works for me, so worth a shot!
I get those every time I put in fertilizer for my plants. It happens to the best of us, a small algae bloom isn't harmful, just displeasing to the eye.