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I recently installed a filter biological and mechanical, and I want to know if the thing growing is beneficial bacteria or algae.
Also what does beneficial bacteria look like, what I have is kinda brownish.
Thanks!

You need to test the water for stuff like ammonia, nitrites, etc to know if you really have beneficial bacteria colonizing. Where are you seeing the brownish stuff? I think the bio media will mostly be clear and a little slimy.

The beneficial bacteria is going to grow - nature ensures that happens. The brownish stuff you're seeing is probably a diatom (alage) bloom. That's completely normal for new tanks and filters. There's an abundance of stuff in the water that serves as food for diatoms. The brown will go away after a couple of weeks. You're best to leave everything alone and start testing the water as Steve mentioned for ammonia and nitrites.