How do I actually know when my water tank needs cleaning — what are the warning signs?

Because you can't see into the tank, you judge it mostly at the tap plus the calendar. The real warning signs are: a musty, stale or off taste and SMELL in the water; water that runs cloudy or carries visible sediment or particles (especially first draw of the day); a tank interior that looks slimy, discoloured or has sediment on the base if you can safely inspect it; and simply a long gap, more than the recommended interval, since the last clean. In Dubai's heat the inside of a tank grows biofilm and collects fine sediment faster than in cooler climates, so problems build quietly before they reach the tap. How bad does it get if left? Sediment and biofilm accumulate, the water quality drops, and in the worst cases you get bacterial growth, this is genuinely a health thing, not just aesthetics. So don't wait for the taste to become obvious. If you're seeing any of those signs, or you can't remember the last clean, it's due, this thread on how often a tank should be cleaned and whether it's required covers the interval side. A proper clean by Al Ghubaiba drains, scrubs, disinfects and gives you a record. Better to be a little early than to find out the hard way.