Wow chuck that tank is stunning. Yea any time I get RTN I usually trace it back to alk swings or low alk, calcium. Sometimes it happens weeks before I see RTN and I'm not sure. It doesn't seem to take much. That pond is so cool I need to put one in at my new house also. The property is sloped so I will have to build a retaining wall at lower end. You should drop a channel catfish in that thing! HUGE