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Does anyone know what these spots can be? they seem to mostly pop out where there is alot of light. Ive had this aquarium for about 3 weeks and got a fluval 405 last friday. they only started appearing since i installed the new filter.

Research told me it might be brown algea (diatoms), but apparently they thrive in low oxygen and low light, which is the opposite of my tank.

thanks!

i'm pretty sure that is actually algae. it's the same thing that's on the glass beside the rock. By low-light, the description probably meant anything besides direct sunlight. I have a mercury vapor bulb, and I had those blooms at the bottom of my tank until my refugium full of live plants fgot established.

Try using an air stone to increase oxygen concentration in your water and reduce algae growth.

looks like algae to me too.

Looks like a tank waste and algae combination. Just clean the tank and it will go away.