Phil
get the fliter manual in front of you, find the diagram where all the parts are listed by No.
item No. 16 - tray.
Item 12, 15 x 3 will all fit into item 16.
Now is cleaning time for filter unit, you start to strip down the filter and you get to items 15, once you have rinsed the media + item 15 in old tank water - what are you going to do with it, where are you going to place, because it still wet - put it into the tray item 12 that what the tray for. Any surplus water being emitted for items 15 will drain into the tray and not spill onto the floor -
Regards
Paul.