What are people using to close off their overflows? I just spent half an hour getting my mandarin out of my overflow for the second time in as many months.
I also use gutter guard. I cut it to the right size and then zip-tied it at the corners to create a perfect square. It's simple to remove and clean as well.
Well... I'm an "old salt" and used what was available "back when". Personally, I like the idea of gutter guard better. It is more flexible, and is also probably a better product to withstand the issues regarding UV degradation. Heck, the stuff was made to be on the roof, under the assault of the sun. I've found that egg crate / fluorescent light diffuser tends to break down and get brittle fast, under the intense lighting we use in reefs.
The only clarification I'd like to make (for those who may be new to this) is that you DO NOT want to use gutter guard or any product made with metals for this. You want to use the plastics and resin products.
This is what I did. I wanted this to keep light out (to hopefully keep some things from growing in there) and to keep animals out (It must be such a pain to 'fish' someone out of there!)
Top
Bottom
Side
Installed
I used a piece of black acrylic that I ordered from McMaster-Carr , some eggcrate, and zipties. The lower eggcrate level fits inside the top edge of the overflow and under the upper tank molding (typical AGA tank and megaflow). The middle eggcrate layer is a little wider to set on top of the overflow but is smaller in the back where the tank moulding sticks out. The top acrylic layer fits over the tank moulding. I cut the eggcrate to fit over the return and standpipe air intake.
The most difficult part of this was cutting the acrylic. I tried a dremel and hacksaw but the stuff is just too tuff. Of course the table saw worked fine but I have a rather coarse blade that even has a few missing teeth so it wasn't the neatest job. And I definitely should have worn earplugs even though it only took a few moments to cut. The acrylic was easily the loudest thing I've ever run though the saw. At least it was easy to drill.
It fits great. If you're wondering, I'm sure you could skip the middle layer of eggcrate, but my return piping is just a little over the top edge of the overflow. Also I thought the middle layer of eggcrate might be useful if my overflow intakes would start to clog for some reason, then it would still be easy for the water to flow in through parts of the eggcrate.
icon under their name to add to their reputation. 
I'm an "old salt" and used what was available "back when". Personally, I like the idea of gutter guard better. It is more flexible, and is also probably a better product to withstand the issues regarding UV degradation. Heck, the stuff was made to be on the roof, under the assault of the sun. I've found that egg crate / fluorescent light diffuser tends to break down and get brittle fast, under the intense lighting we use in reefs.
What ↑HE↑ said!
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