why don't you want to fully submerge the heater? Its how they are designed to work!
on the type of heater gazzas got its only the glass section that gets submerged anything above the glass as to be kept out of the water .
Buy a proper one then! only £15 on ebay for a 500w one,...
how long do they take to heat the water say a 500l tank ? what cost are they to run per hour ?
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for Gazz, why not just buy the right heater for the job or at least a water proof one ?