Silicon will not bond to plexiglass like it will to glass --- I just built a 24 gallon sump with a product similar to weld on (got it locally don't remember the name off the top of my head) --- comes in a small jar and I put it in an applicator bottle with a really really thin needle -- comes out like water but actually sort of melts the plexiglass parts to each other, you can also see it fill the gaps (assuming you have a nice flat surface) and if you pull on it early you can see how it strings apart. Thats the plexiglass bonding to itself. I wanted about an hour per portion I bonded together and gave it a full day to cure. It will keep curing for about a week getting stronger all the time.
I set up my sump as a quarantine tank temporarily and it's filled to the brim no leaks.
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90 gallon with 26 gallon sump/refugium, 2 x 250 MH lighting, 100lbs live rock, 1 - yellow tang,1 damsel, 1 - cleaner shrimp, 1 - coral banded shrimp, 3 - emerald crabs, 2 - porcelin crabs, 6 - peppermint shrimp, 1 - green ricordia, 1 - toadstool, 1 - brittle starfish, 1 - sand sifting star, 100's baby feather dusters,Ass mushrooms, frogspawn, xenia, 1 clam,
6 - Leather coral, Green star polyps,Lime green/orange button polyps,1 huge carpet anenome,various snails/hermit crabs | 
plexiglass adhesive 
"Commit it to the flames: For it can contain nothing but sophistry and illusion" -Hume
