Does anyone else who uses the Dumaguete water supply have problems with rust building up on ceramics (especially the toilet bowl)? I have never seen any postings on this forum on that subject. I can see why iron would be in the water but perhaps if no-one else has the problem it may be due more to my water tank being rusty inside (all pipework is plastic). Some may even have ideas about removal as currently I use: pumice and metal scrapers (but am wary of degrading the ceramics over time), vinegar with additions such as sodium bicarbonate or table salt, manufactured rust removers (which often do not work well).
My brother lives in Junob which is fed with Dumaguete City Water. His toilet bowl is a disgusting mess from the iron. Your problem may more than likely be from the water supply and not your pressure tank. My toilet bowl gets black streaks (sulfur ??), my wife uses sandpaper to remove it. She has been doing this for years and there is no apparent damage to the ceramic.
It has to be a good thing to have filters within your water supply although they are unlikely to filter out rusty water? up in Valencia I believe the water supply comes from a river as it has leaf particles within it and often very muddy after heavy rain, we have two in line filters there, one is just a plastic cage which can be cleaned, the other has a 5 micron paper filter this requires renewing every 3 months however they can be had on line for around p100 each in a pack of 5, these keep the water running crystal clear. This reminds me of the old Scottish farmer doing the rounds on his farm, he was to notice a backpacker drinking from a stream there on his land, he shouts at him; Hoots Mon! dinnae drink that waater its foo O hoss p!ss an coos sh!t. So the man replies to say; sorry Sir I am an Englishman coming here to visit your green and pleasant lands and I do not understand the Scottish vernacular, can you please take your time and repeat what you said using the queens English. Aye richt thinks the farmer? cup,,, both,,, hands,,, together,,, you,,, will,,, spill,,, less,,, that,,, way.