chimera
You have a large tank... can't remember the liters but want do you use besides zeovit. Reason I ask is I have just started using it again in my new tank 800l, tank is 4-6 months old. My last tank used zeo and I did not have the problems I am having now. Mind you I used ASW and did more regular water chages while now I use NSW and do 20% monthly, except this month due to the weather.
Been using Biodigest and my problem of gunk on my sand has gotten worse. Oh never vacuumed the sand but will soon which might help.