I agree w/lotech, need more info on what type of vortex system your interested in.
Rainwater:
1) Are you in an industrialized area? Acid rain could be a real problem. Heavy metals.
2) Agricultural area? Airborne fertilizers and pestisides.
3) A lack of trace elements in rainwater would have to be addressed.
4) Collection method: Rainwater from roofs is contaminated by deposits from above resources, decomposition of roofing material, plus bird and possible animal waste, gutters and downspouts collect leaves and other detrius amounting to = drastically changing ph.
5) NOW, If water is expensive or has poor features in reference to fish keeping in your area, collect a few gallons of rainwater (by the same means that you are going to use to collect it for your aqaurium needs), in a clean vessel, find a laboritory in your area that can test it for most everything harmful to your cause (hopefully at a reasonable cost). Ask them how they want the sample , then give it to them in a sterilized container anyway.
Meanwhile separate rest of the collected water into several clean labeled containers. Place one quart (liter) sealed in the refrigerator and one quart (liter) sealed in the house out of direct sunlight (tjese are control samples). Place one sealed quart (liter) and one unsealed quart (liter)out side in direct sunlight. Now, daily, note any and all agae growth. Use a magnifying glass for this. First two days you may not notice any growth. Third day should start some growth, especially in the open bottle. By the fifth day it should look like you wouldn't want to drink it. If there is algae growth by the fifth day this is good and means you may have healthy enough water for your fish. If you have alot of algae by the third day it could mean a high consentration of phosphates. If all is well you can set up a twenty gallon (80ish liters ?) tank as you normally would and cycle it with some sacrificial fish. Biggest problem with rainwater is the chamical make-up can change from day to day (depending on which way the wind blows).
The above really isn't difficult to do.
I'll keep my fingers crossed for you
Dr Joe