How can I keep mosquitoes away from my collected rainwater?

Water needs movement.

https://www.gardenguides.com/120694-prevent-mosquitoes-rain-barrels.html

The product mentioned here is a nontoxic product. I am assuming you want to keep mosquitoes out of a rain barrel.

You could try affixing a lid to the barrel to keep the mosquitos out, but you'd have to remember to open the lid when it starts raining and you want to collect more water.

My parents maintain a rainwater barrel on our small family farm, and simply dump out all the water whenever it becomes too green and nasty, clean it, and set it back in place under the gutter overhang to refill at the next rain.

Our rainwater barrels have screened lids; we add mosquito dunks once a month, and water our veggies with the collected rainwater.

Keep a goldfish in the barrel to eat mosquito larvae, or add a tablespoon of dish soap or ½ cup bleach to the water inside your barrel or add anti-mosquito larvae, available at garden centers and drain your barrel water every one to two weeks.

Hi Jerry, this is Peggy. If the water is moving or covered it is less to attract mosquitoes. We used to put baby oil in out ditch out front of our home to prevent mosquitoes from hatching and breeding. If you use the water to drink please be careful . Good luck

A bit of vinegar works. Are you collecting the rainwater?

Mosquito Dunks or bits non toxic non chemical can be used in fish ponds anywhere also great for fungus gnats in soil/house plants. Mosquito Dunks and Mosquito Bits are made from naturally occurring soil bacteria called Bacillus thuringiensis strain israelensis (for the rest of us: Bti). They are one of the most effective products on the market today,put in corn cob pieces. they don't kill adult mosquitoes. They kill larvae, but adults will still fly in from elsewhere. Dunks work best if you have a few known sources for local mosquitoes, like a fish pond, small wetland area, watering trough, rain barrel, or unused swimming pool.Plant safe.