I really need help .think this is bad!

I have a 40 gallon tank (still cycling .ammonia @0.25) after 3 weeks .I have 2 mature angels and 2 gouramis in Thére and a polyetherus(berchir) that i bad no choice but to To put Thére for now. Now today i changed some décorations in the tank and when taking thé décorations out .i could smell a foul egg /sulfur odor ...the declorinator i use already has à sulfur smell To it. Can it be that? Also i have a brand new (sicce seachem tidal 55 filter )on Thére and it indicates that thé filter is full after only 3 weeks .so i open lid of filter To find black looking guck on my biobag..what can this be? And is Thére a way to get rid of the smell?? I also realized that my live plants are full of dark brown dots .can it be from the filter?

I would start with a water changes 25-30% a day for the next few days. Maybe some one else will have some ideas. Oh and vacuum your gravel to get rid of old food and excrement etc.