I have started on the mamoth task of cleaning up a rio 400. I have removed the Juwel filter from the middle of the tank and there are 6 huge blobs of silicon on the glass. Whats the best way to remove it? I have tried a plastic ice scraper but its having no effect.

just got to get rid of the build up of salt, white barnically sort of patches and the general gunk. Dont you just love second hand tanks sometimes lol