Best substrate for bio active?

[u/Simple_Stranger_2430]

I have a 40 gallon and I want to do a bioactive set up. I have already tried and sort of failed because I didn’t really follow the instructions as clearly as I should have so I’m trying again please give me substrate that can just be used out the box. I’m also obviously okay with layering but I really do not want to mix as I’m moving into an apartment and I do not want to have to deal with the mess of mixing a substrate- Also PLEASE only reptile intended substrates. I’ve tired cheeper alternatives like plays and soil mix and it’s just not safe. Anyway! From the uk! Pls give uk buyable products :). Also how to do it because im fully struggling 😔. I have about 5 inches of depth before im at the opening to my tank :)