In Saudi Arab, they have this kind of aerated clay brick. Very effective.
Even at mid noon can sleep in hotel room on high floor with just fan on.

But not sure if can get locally. Or if local sifus know how to build with this. Much bigger size than our standard brick.

The house, what happened to the house.......

Plumbing & electrical?

Seriously, need so many rainwater tanks if just to flush toilets and supply a fishpond?

They are not easy to maintain. Without chlorination, all sort of things will start growing inside if the collected water is not used quickly.

Not to mention you are adding unnecessary load to your house, and the unnecesary risk of flooding.

What billyboy sees in his tanks are probably not mosquitoes, but chironomid flies. They die after laying eggs, so you see loads of dead ones. If your tank have them, it means the conditions are suitable for mosquitoe larvae to breed. Better check regularly.

I am also planning a rainwater harvesting system, but one 300L will be enough for my purpose. It will collect form the main gutter pipe, and used to siram bunga, wash the garden etc. For my fishpond, I still plan to use tap water treated to remove residual chlorine. Rainwater is acidic in my area. The size of my pond may not be enough to neutralize that effect. Don't want my fish to suffer.....

UV has very short penetration range, and is suitable only for point-of-use last step treatment, when a small amount of water flow directly next to the light source. Not useful at all for big tank.

The uv lights in aquariums are actually full spectrum fluors for plant growth. You want a healthy bacteria community in an aquarium, you don't want to kill them.

Simplest way will be to drain the tanks regularly and avoid bulid-up of organic matter which encourage bacteria to grow. If need to store the water for extended period add chlorine sparingly.

Rainwater is indeed clean, I will happily bath in freshly collected rainwater, and will even drink fresh rainwater collected directly in a clean glass. (provided this is in kampung area, no acid rain problem.)

There is just an issue if you need to store them for extended period (e.g. > 1 week). The organic matter is the water (bird poo, dust, dead leaves etc) provide food for bacteria growth, which will start off a food chain. Insect larvae etc will feed on the bacteria etc. etc. This happen very fast in our warm weather.
Most of these will be heterotrophic bacteria, generally they are not likely to cause disease. If you don't bath and drink OK.
Algae is not a problem if you close your tanks and no sunlight.

Worst case scenario you get foul-smelling water that might cause a rash to people with highly sensitive skin. And you might see some larvae of sorts swimming in your toilet bowl after a flush.

And you will likely be storing, since your peak collection is >12000 liters, enough for > 2000 toilet flushes. That's two months worth of pooing and peeing.

Installing a UV flow cell and a prefilter is a good option, if you don't mind the cost of extra plumbing and tanks. Make sure the uv cell is properly sized for the correct flow rate.

Otherwise I will just drain it regularly and add chlorine once a week if I need to store the water. Connect a huge shower in the garden and play with the kids.

If you want to use for laundry best include another filter in the laundry line. Fine particles and rust can do hell to white cloths.

If I do this, my actual worry is the extra 12000 kg sitting on my roof. But then again, condominiums probably have 100,000 liter tanks on their roofs.

Too many opinion confusing you.

Your idea is OK. So just go ahead.

If you dun mind the capital outlay, install a uv treatment, gutter shield etc. Guarantee cantik.

If dun want to spend money, just spend a bit more time to maintain.

My opinion worst case scenario you get some not nice smelling water with a few larvae, but perfectly OK to flush toilets.

I got 4 kids, wife planted loads of don't-know-whats, 4 midi -fish ponds, wash my own cars - monthly water bill seldom exceed RM10. Now I pay Syabas RM50 beginning of January, until Dec no need pay, got credit somemore.

Water is cheap, but I am also installing a cheapo rainwater harvest - just to remind myself and the kids not to take anything for granted. And of course, it's just fun and gratifying to do these sort of stuff.

Will start with one 300 liter tank, sitting on a concrete bench 3 ft above ground behind kitchen. Water collected off gutter from ~1200 sf of roof, and will be used mainly to water my wife's herb garden and other plants using irrigation pipes and gravity flow. And washing the car porch.
The concrete bench is to house my two gas tanks, might as well make it real strong and plonk the water tank on it.

Will fabricate most of the stuff like first flush valve DIY. Long term plan if lots of rain, plan to install another tank and a pump to pump the water up to main tank, after chlorine treatment and 3um filtration.

Won't break even in my lifetime, but, you know, fun stuff to do.