Tank water is got filthy over night?

Ok so I have a 37 gal which is currently home to three baby lionheads, one pearlscale, three plecos, one bamboo shrimp, and about three mystery snails. My dad got me two new fish just a couple days ago (one lionhead and the pearlscale) and before they were there, I had two baby koi in the tank but they harassed the pearlscale so I have them in a separate tank while I am fixing the pond outside. I know that goldfish tend to be a messy breed, but I hadn't had any problems before, especially considering the fact that before these two new arrivals came I had three goldfish and two koi in there... (I have a butterfly goldfish quarantined for ick...) I have had the tank set up for a while now. Maybe two months? And all the decor and gravel used in this tank was used in my previous tank (accidentally broke while trying to move it) so I know the decor and gravel isn't to blame. I'm not overfeeding as I only feed them flakes once a day and a piece of sundried seeweed before the lights are turned off. I would try cutting back on the amount of fish in the tank but as of now, all my other tanks are being used. The cloudiness is milky white in color. I have three different plants in the tank as well as a bubble wall and two bubble dragon decors. So I know lack of oxygen isn't the cause. Any one have any suggestions? I checked the tank and no one has mysteriously died and I had just siphoned the gravel and did a 30% water change the other day.