Your storage has probably ...

Your storage has probably gone septic and needs either microbiology or another type of mechanical treatment. Our process would be simple and would help support some level of Dissolved Oxygen (DO). Look for facultative bacteria and you may need to install aeration or simple turnover/flow to ensure the tanks are moving the water as a first fix to clean things up. Very small flow, but we could give you both biological fix or mechanical fix.  Once cleaned you can probably suspend mechanical, or cycle, and operate mainly on microbiology.