Belly, hope the below information could help.
I think the things you have done caused the troubles: using new plastic black cotton gate; removing the moses out; changing water too much, using tap water to clean your filter material. Anyway, you are the only one that could really deduce what happened.
Back to basic :
1. Nitrobacteria colonization is the vital vital element;
2. Tap water contains flourine that hurt fishes, shrimps and KILL YOUR nitrobacteria;
3. Ensure your filter material keep longest contact with water, with largest contact area.
4. What is the function of charcoal?
Using tap water to rinse your filter material is murdering your lovely nitrobacteria. Next time, just pour some of your tank water into a small bucket and rinse the filter material SOFTLY. The aim is to keep sufficient nitrobacterial colonization. Otherwise you will remove/kill them all and you will have next cylce of milky water.
I guess I know that black cotton gate but I never use that before. Therefore, I cannot give comments on its effectiveness and efficiency.
4. There are still a lot of debates on the use and function of charcoal and how it works. If your filter system is good enough, you can flow away those charcoal. I am not qualified enough to talk over charcoal. Please search within this web or look at the following link:
http://www.marineland.com/articles/17RevisActCarb.asp
Cheers,
Kelvin6666 (but not a bigbrother

)