Cistern critters?

The main issues I have with the preassembled UV system Gallows Bay Hardware sells (which is not a bad system) is that they do not have unions to facilitate removal of the UV unit when you need to drain it to inspect/replace the quartz tube the UV lamp sits in and they have a filter housing installed downstream of the UV lamp, which is against industry recommendations due to the potential for bacterial contamination of the downstream filter. The last filter is typically a charcoal filter which can be a good breeding ground for bacteria. When you lose power, the UV lamp goes out but the cistern pump bladder tank will still be pressurized, If you flush a toilet or are taking a shower when power goes out then water will still flow through the UV chamber without being exposed to UV light, and potentially contaminate the filter. If the filter is downstream of the UV lamp and becomes contaminated, then any bacteria growing in the filter will never be exposed to UV light in the future, which is the reason for the standard recommendation that the UV lamp be the last component in any UV purification system.

I agree JE,
My plumber installed the 2 partical filters, then the carbon filter, and then the UV filter last in-line.
The display at Gallows Bay is incorrect.

"I haven't cleaned my cistern ever"

What kind of timeframe are we talking about here?

I've owned the house for ten years and its doubtful the previous owner ever cleaned the cistern either. The two cistern chambers have thick vinyl liners. You can't just empty them and shovel out the silt. My water is clean, delicious and tests well. I am not concerned.