I shared a google slide presentation an calculator on Rainwater Harvesting. It's an intro course. I hope you'll find helpful.
Be sure to check out the collection/storage/use sizing chart slide. Calculator is also here in case the url isn't working in the sizing slide linked above.
Use at least one first flush diverter. The idea is to collect the cleanest water possible before storage. Then you want to "use" the cleanest water from the tank. So build a floating intake yourself. Bad for gravity use but good for quality. I use gravity for a backup distribution system so I flow from the bottom of the tank.
If you don't want to risk getting sick, consider a 9 and 3 micron dual-stage filter with uv light. Use large, sturdy screen at the in and out flow of the tank to keep critters out. Chlorine at 1 part per million will kill pretty much everything and I think most instructors recommend some combination of all this stuff.
I recommend using everything. Especially if you use pesticides that could be ingested by critters. When they get sick, they look for clean water supplies.
Get a part per million meter to gauge how close to "perfect water" you can get.
Get a ph meter to gauge the acidity of the water.
Addendum for the pluviophiles (lovers of rain):
I live about 7 miles from an active volcano. My metal roof flows into plastic gutters and pvc pipe is used to convey the water to my food-grade holding tank. It has crud on the bottom ie; life.
My water has a slight yellow tinge from the Ohia tree leaves that collect in the gutter. I give the water a bunch of positive intention, specifically love and gratitude and the color will change, sometimes.
I use a first and a second flush diverter. I don't use filters and do very little mainteance on the system. I have a close connection with the water.
Ultimately, I believe the connection you develop with your water is a
big health factor. It's worked well for the during the last 5 years.
Some folks see rainwater harvesting as owning their own water supply. I see it as a gift to be able to attract from the sky, one of the things I need most. I REALLY love my water. It's the only water that quenches my thirst. Bottle water makes me thirsty, tap water smells and tastes too much like chlorine and eh, I forgot what else I had to add. Did I miss anything?
Also watch the "Rainwater Harvesting TED Talk"
Ka Wai O Kane!
(the water of Kane, or rainwater)