Of coarse my go to is 3D printing. Filament I use I know is reef safe and I also just bought myself a Christmas gift of a decent resin printer and cleaning station that I hope to set up Christmas week when I am off.
Use that plastic mesh that yarn people use to cross stitch. No idea what it is called but I have used it a few times to make things. Just make a small basket or something with some tiny zip ties.
My thoughts are about how water soluble the filaments are. I printed a jar at 100% infil .1mm and it still leaked water absiolutely everywhere, and if I don't keep the filament in a dessicant container, it can get really stringy when printing, so I just don't think they're good long term in the tank (I think they're made of corn, so as long as it's made in a reputable shop it should be fairly harmless). I think most of the PLA is biodegradeable, so it likely won't last with running water passing over it. ABS might work, but I have yet to get a successful ABS print, as I use those print mats and don't want to be spraying them with adhesive...
You think the resin prints would be reef safe? I was watching a print yesterday, and MAN those things put out some toxic fumes! Damn neat prints, but the mess and smell means I'd have to have a heated outside shed or a fume hood to use it.
I tried a mesh over the intake already, it filled with snails and whatnot and got blocked, and got sucked in as well, although I did have it loose and not tight, but I'm toying with gluing screen over the opening. These snails are insane. They seem harmless (they sure don't eat zoas), and they leave my clam alone, but man are they annoying
