Jack sort
Aquarium Advice Regular
How much bio media was in the first filter? I'm guessing not enough. When you added the second filter and got rid of the carbon from the first, you may have lost a bit of beneficial bacteria that was housed on the carbon. And are now experiencing a bacterial bloom. Do large water changes until your filters can catch back up. And then continue to do large water changes forever because your fish will appreciate it. The more and bigger water changes you do... The closer your tank becomes to your tap and the less your fish notice it. If your fish become stressed from large water changes ie.. 50% or so, you haven't been doing enough water changes. The beneficial bacteria is in your filter media not the water column. Oh and watch your ammonia and nitrite levels if this is a possibility in case you are experiencing a mini cycle. Don't want your fish to suffer unknowingly and then all of a sudden you have fish dying.
This is a great point, I never thought about all the good bacteria that was in the carbon bag.