It could be any of the thing's you put in there for him if you didnt rinse the gravel enough, It could be a number of things
If the water is cloudy immediately, or within an hour or two of filling the tank, it's probably due to insufficiently washed gravel Drain the tank and rinse the gravel until the water runs clear. That should resolve the problem.
If washing the gravel doesn't solve the problem, the next most likely cause of cloudy water in a newly filled tank is a high level of dissolved constituents such as phosphtaes
silicates, or heavy metals. If you test the water, you'll no doubt find the pH
is high (alkaline). In these cases, treating the water with conditioners will often resolve the problem.
Those are a couple tip's from the tetra care program also like i stated before it could be the tank cycling i set up one of my 10 gallon tanks and used a fish to cycle it, By the next morning it was cloudy and had started cycling.