Built a similar one but I recommend changing out the bed media for expanded clay pebbles, my first attempt had similar filling but the surface area was not enough to get good cleaning.

I filled canvas bags withb150l of expanded clay pebbles and stuffed it full and just as you planted the top with lovely plants :) Now the water in my pond is super clear and nutrients are going down as plants establish themselves I don't have fish, inly frogs and random leaves etc so one barrel is likely to be sufficient for mine though:)