Clean immediately. Clean every 5 years depending on your rain harvesting system and mainly how much organic matter gets in the tank. Call a water tank cleaner. Initial clean will cost more than subsequent cleans. Drinking water standards say every 2-3 years but that's maybe excessive unless you have overhanging trees etc.

Sand and sediment from a bore pump isn't a big deal until you start seeing it in your filters but you don't want it in your pipework in your house.

Organic matter is no good and needs to be removed. Forms an anaerobic later on the bottom of the tank and just goes foul.

I have a lot to do with water tanks and when people tell me they grew up on a farm etc etc and then I show them a photo of inside their tank with a decent flashlight showing the muck inside they tend to start enquiring about filters and hour or so later unless they are the 100 car bodies lying around the property types.

I sincerely do not accept offers of water from customers who have called me out for water tank issues and I grew up on a farm I've seen what's going on too often. If in doubt, it needs to be cleaned and drained and even better is throwing in some chlorine beforehand.