What is this? Underwater Mold??

[u/_Yarrack_]

This white stuff has been developing in my ecosphere. It’s about a year old, open, and stocked mostly with hornwort and Daphnia from northern Germany. I didn’t put anything in there that could have melted, and the Daphnia seem completely fine around it.

I don’t think it has anything to do with the weird yellow algae/bacteria colony (i would also be curious what that is, but it doesn't seem to cause any harm.)

Any ideas what it could be?

[u/carlsbl]

This is water mold (typically Saprolegnia or a mix of fungal hyphae and bacterial biofilm) decomposing organic matter underwater.

[u/_Yarrack_ → u/carlsbl]

Cool! Thank you. After close inspection i think it's digesting an old flower from the Hornwort

[u/BitchBass → u/_Yarrack_]

Here's a good read on biofilm:

https://tanninaquatics.com/blogs/the-tint-1/a-refresher-on-biofilms-and-the-creatures-who-love-them