Have you done anything significant in the last 5 months leading up to the start of when you noticed the milky cloudy water? I'm thinking it could be a bacteria bloom and that usually occurs when things are thrown out of balance. Perhaps you added in too many fish? Or changed out all the filter media and sponge which can result in the loss of too many beneficial bacteria in the process? But if it's milky, most likely its a bacteria bloom. If there's a greenish hue, can be an algae bloom, which means too much light. I'm grasping at straws... LoL
You can do some 20-30% partial water changes the next few weeks (2-3 days apart). It should clear up on its own. Just don't overfeed and you can reduce your lighting.
If it still doesn't clear after the above suggestions, you can try a water clarifier agent. It basically makes smaller particles floating around clump up info larger particles in which your filter can suck up. After the agent has had some time to work, then you'd have to do a gravel vac and siphoning to get the gunk out.
Hope this helps.. Good luck!