Physical scrubbing is the obvious best method, principally because you must 'visualize'.
When first using the "tank cleaner" I naturally had 'snake oil' trepidations;
but, I have prolonged/doubled my cleaning interval from every year to two years with the Startron product ... as I find that it doesnt 'loosen' the debris deposition; rather, it seems to slowly put the crud into 'solution' (& without plugging the filters) and that does takes time. I do semi-continually operate an onboard polishing system, so my large tank doesnt have the chance to become as fouled as yours, even after being in the 'tropics' for past several years.
Ive also started using the 'tank cleaner' in my multiple home heating oil tanks ... and yes, it does seem to be slow acting but is positively reversing a 40+ year old very thick deposition layer and without clogging the filters or flame nozzle.
With respect to the 'cleaner stern', I would expect that too as most enzymatic action makes 'smaller' particles which would have an enhanced probability of being 'more completely burned' in the combustion chamber, rather than 'surviving' the combustion chambers and being deposited in the exhaust leg.
Suggest you give the 'tank cleaner' more soak time and if possible do this in the heat of the summer for the thermally enhanced reaction rates. Im still in the 'learning curve phase' of using such tank cleaners and so far my observations are it does take a quite a bit of time but doesnt have the disadvantage of 'loosening' a LOT of crud which would create a 'particle storm' of the larger particles. More patience!
Last edited by RichH; 01-09-2011 at 08:09 PM.