It is a relatively new tank to me....fully cycled. The filter was a slimy green gross mass when I got it and I cleaned it lightly with tap water.....I have a well so chems are not a problem...even so I have tested and it is clean.
I did not completely clean the filter, just enough so water would flow through. Also, the Tetra brand filter pumps have a separate screen mesh for bacterial growth so that you can change filters without having to re-cycle the tank.
Based on the chem testing the levels are all fine....but the tanks is hazy and stinky.
My well water is VERY hard...over 400 tds
most of which is calcium. I use softened water for about 18.5 gallons and hard well water for the last 4-6 quarts. I figure the hard well water could make it hazy, but there is very little hard water in the tank.