I found the same thing in my pond filter. Closest I can ID them to is Bryozoan. Tiny, sessile, multi-branched, filter feeding organisms. In the wild the colonies can form large "blobs". They are probably enjoying the current provided by the filter.
Oh no! Now I feel bad- they are ANIMALS??!!
Fresh2o thank you for finding that for me, I never would have found that out by myself!
These guys are in the filter of a tank that I have been battling a number of diseases in. They must be tough- I even treated that tank with copper!
Some can be large enough to see.
If you can support organisms without meaning to, your fish are probably doing well. XD
Take it as a sign that your tank is a thriving ecosystem:thumbup:
Sent from my SAMSUNG-SGH-I337 using Aquarium Advice mobile app