ive had something similar to this before one was due to an overpopulated tank with large fish after a water change theire was a nitrate imbalance the water turned so grey you could not see threw the tank 2 fish survived.
The second time this occured i changed fish food wich contained some toxins this time the water was green and killed the entire tank
I got my sump custom built to perfectly fit in my cabinet doors etc. What I didn't measure was the height of one of the downwards protruding beams that support the tank, I only measured the door and left a reasonably gap.
i bought a 300 gallon stock tank for my basement about 4--5 yrs ago. only problem is that the basement stairs dont go just straight down but they make a right hand turn. the stock tank was to tall to make the turn. i had to use a saws all to cut the top 12 inches off then made it through. silicones the cut off piece back on lol i was bummed i had to do that though.