Why is my black water tank smelling so bad?

I’ve recently setup a backwater tank for my betta and it keeps developing this thick biofilm of the top of it and it just constantly reeks, I have no clue what started this I’ve kinda just left it hoping my plants would cycle it better but it hasn’t gone away for about a month now. I’ve tried doing partial and full water changes and it always ends up coming back so I have no clue what is necessary causing it.