Help! Stuff growing on water surface! (tadpoles)

[_Jai_]

I have eight tadpoles (most likely Eastern Banjo). They're in a simple small tank and I use rainwater.

Recently this brownish-white speckly dust looking stuff appeared on the surface of the water and started building up. I cleaned the tank just yesterday and it's already back in the exact same spot!

Does anyone know what this is? How do I get rid of it?


(sorry it's not very visible in photos, it's just started growing but if you need I'll put another image when it builds up again)

[Autumnsky]

How's the water movement in the tank?