Plugging holes in acrylic

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I have a 180 acrylic tank that i picked up awhile ago. Been sitting in the basement thinking of setting it up. But it has four, 1/2 inch holes drilled in the bottom of it that they ran some weird filter system for fresh water. Just wonder what the best way to plug them would be. THANKS!

 

[JCR's Reef]

You can buy bulkheads and then screw a plug (PVC I think) or you could make small acrylic panels and then glue them over the holes, or run a closed loop system and utilize the holes that are there.