Originally Posted by Andrew McFadden
it probably not cycled all the way quite yet. By adding the other filter without any cycled media wouldn't have help quality but it could add oxygen. Aim the other filter up to the surface of the water to create more oxygen.
It was solely the oxygen. I removed the media from the old filter days ago and thought the ammonia level didnt raise enough for it to be gasping and the new filter doesn't disturb the surface at all.
So I bought an air pump. It's been running with the new filter for the last day or so. No problems yet! I'm still watching it, but it would seem it was a lack of oxygen.