I have this white residue stuff that is on my tank. It's a 10 gallon tank and only has 1 betta in it. It's been set up for 5 months about.
The residue stuff is only at the parts of the tank where there ISN'T water, such as the upper 1 inch of the glass (I don't fill it to the very rim), on top of the heater on the part that isn't submerged, and on most of the parts of the filter that aren't in direct contact with water, and I think there may be some in small crevices in the hood.
It's ugly and I'd like to get rid of it safely, but I'm not even sure what it is and what's causing it. I wiped some off the front and sides of the glass with a dry, clean washcloth so it looks better, but still I need to get rid of the worse parts on filter, heater, and back wall of tank.
Also, when I vacuum clean then add new water, I sometimes add a bit more than what was previously in there (like if some had evaporated), and the residue gets slightly covered, and I'm pretty sure it is coming off the glass and going into the water as I can see small chunks of white stuff floating.
I took some pics of the stuff on the filter (with my camera phone though so they aren't the best), but you can see the white stuff is pretty bad on the filterright above the water line and higher. And in the 1st picture if you look closely you can see what looks like a big scratch on the glass, but its not its the white residue: