Traditionally, inbreeding is used in stock breeding to reinforce desirable traits AND to eliminate undesirable ones. But you have to relentlessly cull bad traits when the genes are reinforced to get improvement. Where inbreeding is not combined with harsh culling (such as when a breed of dog gets "hot" and the puppy mills churn out all they can, or in royal families) we see the negative effects without noticing any improvements (obnoxious hyper poodles, labs with bad hips, and Bruce's horse (faced) picture).
In a pond with lots of competition, culls should do poorly and starve, get eaten, or reproduce poorly due to disadvantages in size or condition. We can hurt or help this process by removing the biggest and the best or culling the small and the weak.