I have similar problem with my Weeksii bichir chilling in front of my overflow box blocking the drain slits. When this happened a few times I find my display tank's water rise almost over the top euro bracing of my tank and my sump almost running dry but lucky I designed my sump with a last chamber of only 15ish gallons of water. Also I have 2 overflow boxes so the sump won't run dry if one end gets clogged by a fish. No matter if both sides gets clogged I made sure my display can handle 20 more gallons of water so it won't flood. Anyhow sorry that these floods happen to you. IMO if the sump is designed properly then u wouldn't run into flood issues.