Lenin, The crucial items ...

Lenin,

The crucial items that will cause odor in your rainwater harvesting tank are as follows:

-Types of pre-filtration

-Collection surface material

-Periods of storage

If you are using the right pre-filtration before the water enters the tank then this will reduce the amount of organic matter entering the tank. Ecoli and other harmful bacterial feed off decaying organic matter. Which then releases a heavy carbon odor that  you are talking about. 

If you are collecting from a "green roof" this will cause the water to pick up a lot of organic matter which in turn makes it difficult to filter and gives the water a heavy odor. Have you ever tried to filter water that was ran through a tea bag. That's the same concept.

If you store water for a longer periods of time then that water begins to build up organic matter and also becomes less oxygen rich. Stagnant rainwater will take on a fishy odor. This is mostly like because you have  over sized the tank.

To fix these issues you can do the following:

-Put the proper pre-filtration in place before entering the tank. I recommend the Wisy Vortex Filter.

-My company has a substance that we use that is natural tank cleaner that will help improved the overall water quality inside the tank. Chlorine tablets are another option but you have to create a schedule for usage.

- Recirculate water or install an aeration system inside the tank. If possible increase your rainwater usage per month. 

Shawn Crawford