Question on water storage

[Spotted]

Ok, so I got a Brute dark grey garbage can for my new salt water storage container. My trouble is when I mix the water for say longer than 3 days, it starts to smell bad and looks like it grows algae.
It is dark and the lid is on tight, so no light is getting in. I use a MAG 9.5 or 3 Korelia 105 pumps for mixing. Am I missing something? Anyone have any ideas?
I want to change my method of water change from every week to 2 weeks to smaller daily water changes with an automated type of system. Problem I am having again is the change water getting fouled.
Thanks!!

 

[Dav3]

Are you using rodi water to mix with? Did you clean the garbage can before use?

 

[Spotted]

Yes I use RO/DI, and yes I clean the can before and after every water change.