I designed my sumps overflows like that and always found loads of baby shrimps in my sump crawling around in the biomedia. I placed a small piece off coarse filtersponge on top as a cap. But still the baby shrimps sneaked in.. Quite a hassle to get them out again and back in the tank. They don't survive for long in the sump biomedia. Now i redisigned it with taking apart an existing filter inlet,
took off that basket at the end and placed it upside down over the overflow, i was lucky it had the excact same diameter.
It solved the problem completely..
But
@kiddjam i think i know these 15mm rubber bands you used, a friends daughter has bags full of it and knits them into wrist and neck bracelets and all kinds of stuff. They are pretty popular among the girls and see these bags with rubber bands solled everywhere toystores and even tankstations have them sometimes.. I know they also come in black and grey because she made a bracelet for me as well and it has black and grey ones in it.. Here is an example pic it has a black one, i asked her a few spare ones.
I kinda used them too for alternative purpose because these bands are pretty durable and flexible for their size.
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