I recently purchased a 10 gallon aquarium that came with a pretty basic filter. I was going to stock it with a single goldfish from our outdoor pond that has goldfish and koi in it. Unfortunately I underestimated the size of the goldfish in the pond, and ended up with a fish about 10 inches long, including the tail fin. He keeps bumping into the sides, and does take up quite a lot of space. The filter has been doing a good job keeping the water perfectly clear, but I was wondering if his large size could pose problems down the road.