So my friend had gotten feeder fish(killies) for his tank. They are very small fish. So this is what happened. He never blocked off the u shape hole to the left.
So they went into the drain. Through the piping.
Into the filter sock and caused it to clog and overflow. He was wondering where all the water went and why his return pump was sucking air. He was like dam this thing evaporates really quick. Then His daughter goes into her room that is directly under where the tank is and she was like daddy I think the ceiling is going to collapse. Now he has to replace some Sheetrock and paint. Lol. That u shape hole is now closed off so that will not happen again.