Hopefully can get some help. I built a small koi pond three years ago. No problems. Six healthy koi. This summer I expanded the pond (essentially, created a second pond and connected them with a "swim-way" (about 2ft long, 1ft wide, and 1 ft deep). The second pond is larger and deeper than the original (probably 4ft long 3ft wide--at the widest--and 3 ft deep and its deepest). I also added a second waterfall to the new end of the pond. So now there are two waterfall on each end and lots more room. However, since introducing old fish into the expanded pond (then replacing them with new fish), I can't keep the koi alive for more than three days. What's going on?