Someone left the cover off my cold water tank in the loft. The water surface is covered with white powder and other fine debris, floating. The bottom doesn't look too clean either. I could clean out the bottom easily enough with a siphon hose, but I if use that to clean the floating stuff off the top, won't air go up the tube and stop the siphon working? Any ideas? It's a 77 gallon asbestos tank.