If possible: Just remove the smallest amount of plastic possible that allows you to fit the skimmer over the lip, if you are able to do that without messing up the integrity of the lip. Looks like it’s pretty thick up top there, probably meant to hold a heavy glass lid. Alternatively, get creative and make your own mount (I drilled small holes in the lip of my 28g to put zip ties through in order to mount my ATO hose)
that’s one route, but that’s potentially kinda permanent. The other option is to consider the size of the skimmer, and what you want to accomplish. The Fluval one is rated for 45g, and without a sump is going to be your primary mode of nutrient export. You have a 55g tank, it might be worth sending it back and trying out something larger that fits better.
Your tank is looking great, and I really love the build on the stand!
that’s one route, but that’s potentially kinda permanent. The other option is to consider the size of the skimmer, and what you want to accomplish. The Fluval one is rated for 45g, and without a sump is going to be your primary mode of nutrient export. You have a 55g tank, it might be worth sending it back and trying out something larger that fits better.
Your tank is looking great, and I really love the build on the stand!

