Ideas for using 15,000 L of water (in a tank)?

[Cyberdragon]

Make a hydroelectric plant.

[Ampera]

Why on earth your government is obliging you to have 15.000L water tank? it doesen't make any sense... 

At times Australia can get very dry, and the dams supplying Sydney can become critically low, especially with the stupidly expanding population of Sydney. So there were water use restrictions, raised prices, public campaigns to make people save rainwater and use it for garden watering, etc.

But that was then, and recently the weather is much wetter. And will remain so for years, if Mr Maunder and his spotless Sun has anything to do with it. Currently all the dams are near full:
  http://www.waternsw.com.au/supply/dam-levels/greater-sydneys-dam-levels

Then there's the AGW religious resistance to any suggestion of the climate becoming wetter for decades, because of course all climate change is baaaaad. So there's no rollback of the water use rules and pricing. Maunder glaciation will proceed faster than legislation change.

As for using it...
Firstly the brown stain is tannin, from gum leaves. In concentrated form it can be used to tan leather. I'd rather drink rainwater with tannin, than town water with chlorine and fluoride.

If you have somewhere on your property significantly higher than the house, you could put a second small tank there, with a small solar powered lift pump from the big tank. Then use gravity-fed water from the small tank for most house use, rather than town water. Showers, bath, toilets, etc. Filter for drinking and cooking - remove the tannin, still no fluorides.
Manual valve switchover to town water if there's been insufficient solar pumping and the small tank emptied. Keep it simple.

Insulating the main tank and using it as a thermal mass for house air-con is also a good idea.

Then there's the bushfire drench system. Something I plan to do when/if I move to the country. Not so much to save the house (mine would be fireproof anyway, partially underground), but to preserve surrounding trees. The idea is to spray drench everything in sight, before the fire front arrives, so nothing burns. Obviously, must be a diesel driven high capacity pump, since the town power would probably be out in such a situation.
If you already live in a moist fernery, there's probably little point to this.

I honestly don't get the mindset of people who are upset about fluoride and chlorine in the water supply. It's not enough to kill you, and you would sooner die of over-drinking and failed kidneys than you ever would any affects from flouride and chlorine. You ingest MORE fluoride every time you brush your teeth, and heck the people on the ISS have to swallow their toothpaste for 6 months, and they are all fine up there.

[TerraHertz]

I honestly don't get the mindset of people who are upset about fluoride and chlorine in the water supply.

If only your google-fu was as well developed as your programmed kneejerk.

http://everist.org/archives/links/__Fluoridation_links.txt
Just in case you are honestly curious _why_ some people are concerned about compulsory water fluoridation, that will get you started.
Chlorine I agree is not a big issue, but I'd still rather drink water without it, given some other way to kill any pathogens.

Oh, also related: One of my hobbies is 'urban exploration', which leads me to sometimes be places where the public is not supposed to go. Which unfortunately I cannot detail here. But I've seen some things that greatly reinforce my anti-fluoridation opinion.

I've read: an entire dam dropped down to 16%? That's actually insane, at these levels there's no way to produce electricity in a large city... Now it's clear that if anyone stores water there's a good chance that during a drought you may find yourself without water...

None of the dams around Sydney have operating hydroelectric plants, the water is too precious to just dump through a turbine. Waragamba dam (the biggest) was built with a single small generator turbine built into the dam base but it's never used so far as I know. Probably only intended for local power in emergency.
Woronora dam also had a tiny turbine generator added around a decade ago, but that too is only for powering dam infrastructure in emergency. I don't know about the other dams.

The major hydroelectric systems in Australia are the Snowy Mountains system (some of the power to Sydney and Melbourne), and the Tasmanian systems (most power in Tasmania.) Other than that, most electricity is from coal and gas stations, with some wind and solar.