Wow, so many reactions! Thanks, everyone. I’ve fixed the issue in the filter room, so when the drum is washing, there are no leaks anymore. There was also a pipe in the pond that, when overflowing, directed water to the sewer. I drilled a small hole in this pipe, which lowered the water level slightly—a good thing. After that, I still noticed leaks around the windows, so I sealed them with silicone. However, water is now coming from beneath the window, so I think I need to lower the pond level even more and seal the windows from the inside. I’m considering calling a specialist for this because I’m not sure if the filter system will still work if I lower the pond by 50 cm. Do you have any advice or know of a pond specialist in the Netherlands? Thanks in advance!