I have hard, clorinated water which is not good to use in a fish tank, so during the rare rains we get here in southern California I try to accumulate some rainwater that comes off my roof. Some tricks I learned.

1. Don't use the rainwater from the beginning of the storm, let the rain first wash the dirt and dust off the roof and gutters for a few hours.

2. Don't leave a container (I use a plastic garbage can) filling up all night. The reason is there is still some dirt on the roof and also in the rainwater itself just from falling through the air. If you let it fill the barrel and overflow the dirt settles to the bottom of the container and the cleanest water overflows over the top. whereas it might take 20 gallons of water to fill the garbage can, over the course of the night maybe a thousand gallons would have fallen into it, and the dirt gets concentrated and you end up with a barrel of dirty water. So keep checking the barrel and as soon as it fills pull it away from the downspout or wherever you are getting the rainwater from and you will have nice clear water ready for the fish tank.