They have found microplastics in babies developing in human females. They are also found in canned sardines because the companies remove the head of the sardine but leave the guts in the fish. The small fish pick up and ingest the microplastics while eating plankton.
Air operated sponge filters are usually made from polyurethane foam rubber, which isn't a plastic but is much more toxic if it burns, and when it dries out it releases fine dust that is pretty toxic too.
The course sponges in some power filters are plastic and they could release microplastics. There are also plastic plants, ornaments, light units, filter casings, most things fish related are plastic (as Magnum Man mentioned).
Washing machines release huge amounts of microplastics from clothing that can be filtered out quite easily but governments won't force washing machine companies to sell or supply filters for the dirty water. Instead the microplastics from clothing go into the sewer system and end up in the rivers or oceans.
Sea birds regularly have plastic items (not microplastics) in their crops and stomachs. They pick them up while fishing, the plastic items reflect in the water and looks like a fish or shrimp. The bird swallows it and feeds its young. Baby sea birds around the world are dying from starvation because their bellies are full of plastic items.
We really have ruined this planet.
