Apologies to all the hardcore aquarium people for calling this a "Fish Thingy". I made it for my sister and am not an aquarium owner myself.

This is a diffuser to prevent a high fluid flow rate filter from pushing fish. Though I don't know that PLA would be an issue, PETG might be a safer plastic in terms of fish safety, though I printed it in PLA and the fish have been fine. This model is far from perfect, but some people asked for me to post it. If anyone wants to improve it, here are some changes I would make:

1. Make the holes increase in diameter from the filter.
2. Tweak the clip geometry (the filter this was designed for had mostly curved surfaces which made tight-fitting geometry difficult to achieve).