As an engineer deeply involved in filtration engineering for over 35 years .... recirculation polishing of fuel
without a thorough mechanical cleaning of the tank is of very little benefit.
Simple speak: If you dont get inside the oil tank and scrub the thick biological deposits from the walls, these 'agglomerated' bio-particles will continue to break off the walls and re-contaminate the oil, plus the cellular debris that does break loose only becomes the nucleation sites for even more growth of larger particles. Without a thorough cleaning of the tank walls, recirculation polishing of the fuel
oil is only a very temporary remedy.
Most tank fouling is usually caused by a fungus - Cladosporium Resinae or 'kerosene fungus', etc., species that use the carbon content of oil as their nutrient source. It usually enter the tank through the VENT. Growth of such species is exponential.
How to know when to clean out a tank: remove some of the tank oil, put it into a clear glass container and hold it up between your eyeballs and a strong white light - if there is any 'haze' noted when you look through the oil to the strong white light, it means that the oil is fouled/contaminated and that the tank should be cleaned .... then the fuel
should be 'polished'.
For the price of a single commercial 'polishing' one can easily build their own on-board polisher: a simple Walbro type 12 vdc transfer pump
, a 10" steel filter housing, a ~15µM 10" X 2.5" dia. industrial resin bonded cellulosic filter .... just continually pump
directly from the bottom of tank directly back to the tank. A good 'rate' for recirculation polisher is approx. 2-3 gallons per MINUTE per 100 gallon tank. If the system is 'clean' that 15uM filter operated anytime the engine is on will produce a particulate level at 'submicronic' levels, the oil will be 'crystal clear' (no haze) .... and there will be no challenge to the system's inline (Racor) filters. Any particles that do get into the tank will be constantly and quickly removed. Such a system is very cheap to use as you wont be changing out your racors all that often as the debris is removed by the 'cheapy' industrial filters in the recirc. loop.
Do NOT put an onboard recirculation polisher in/on the main fuel
delivery system or its return line