| Steve, you might find this interesting- I did and experiment with green water and bacteria house media. The green water clearned by without adequate prefiltration of dead agal cells I was able to induce environmental gill disease. Not a shot at the media itself because it did a fine job of ammonia control. But the lack of organic management/removal created a bacteria count that in this case, did not cause aeromonas but rather gill disease.
I think all these systems, UG, magic sand, pebble filled garden ponds etc . all lean towards overly organic and temptingly anaerobic inclined environments. And therefore are not the right environment for koi- especially adult koi, unless monitored and maintained aggressively.
JR |