To me the 405 has 2 halves. One side is the" vertical" tray that holds 4 foam blocks. The other side has 4 "horizontal" trays that all stack on each other.
The trays can only fit in 1 way, the drain hose outlet should be positioned over the verticle trays that hold the sponges. The return hose where the pump is should be positioned over the small stacking media trays. The top tray will have a lid which fits snugly into the return pump inlet on the top half of the canister. If anything gets mixed around its not likely to seat and seal correctly.
I used to love canisters. Lots of screwing around. Life is busy, I'd rather mess with a sump or a hob filter.
Fluvals are pretty user friendly. Once you get it down it won't be a big deal. Maybe check some youtube videos? I had to when I couldn't get my eheim to prime
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