simple rain water harvesting can do for a few hundred ringgit. but the quality of water not so great. just use for watering plants.

the issue is water is quite cheap in Malaysia.

if you are a true nature lover (irrespective of cost), consider filtering the rainwater (which adds up the cost; first flush, filter), storing in the tank, and use for TOILET FLUSHING (much like HK apartment use filtered seawater for toilet flushing).

Commercially, it does not make sense since treated water is cheap, so only if you are a true nature lover.

i have more than 10 160l tanks which i use for my own rain water harvesting.

the easiest way to ensure no mosquito (think Aedes) is to keep it covered. everyday, when i open it there are loads of dead mosquitoes on top of the water. i think the mosq eggs are everywhere and when they hatch, and there is a cover on top, after some time they crash and drown in exhaustion.

consider putting a simple wire mesh (small enough so that the mosquitoes can't fly off) before the storage tanks. cheap and cheerful solution.

i have surveyed the 'proper' way but it cost several thousand ringgit.

your fish will love the rain water. its much more refreshing than tap water. in fact, your fish will also love all the dead mosquitoes on top !