After surviving a dose of giardia, I have used Pour n Go on a yearly basis for general disinfection, especially if there is no filtering.

But if the water smells like something has died in it, then a drain, clean and refill would be in order.

Rats can get stuck, and die, in the gutter and drainpipes. Makes the water taste funny.

So... that photo on your profile - is that taken after tasting the 'funny' water?

Watercare in Auckland have a commercial test lab.  You could try them.

If I owned a rain water tank I would be really sure that the lid was very securely closed.

Our farm had a 7m high maize silo (finish fibreglass hopper with steel framework/ladder).  Possums could climb up there.  They have no problem climbing up any water tank and if they have been poisoned (ditto rats and mice) they are attracted to dark places and/or water.

I find yearly water tests a useful pointer as to what we're doing right (or wrong) with our system.
Noticed things like there's been a change in our bore feeds' soluble ferrous Iron with a rise in pH and increasing Chloride (likely salt water intrusion, we're close to the ocean)
It'd get pretty expensive if you're testing to all New Zealand water standards.. we test for the most likely contaminants at a cost of $2-300 per time

We test both our bore water feed and clean water (post treatment) tanks for pH, EColi, Colour, Turbidity, Hardness (mg/l), Alkalinity (mg/l MG) Calcium (mg/l CA) and Chloride (mg/l CL)
Rainwater feed tank inlet basic test for pH, EC, Dissolved Solids etc (pH does rise the longer it's stored in concrete tanks)
We've used several different companies over the years for annual water analysis reports.
I think any IANZ Accredited lab that lists water testing should be able to do the job. We're now using one off this List

One close by is important as readings change with increasing time and warming sample water.
We also regularly do our own clarity & smell tests (heat the water slightly & have a sniff!)

Pool chlorine will do.

It's 10grams per 1000 litres.

Of course Watercare make you pay Fixed charge: $647per year for your tank....

SepticSceptic:

 

After surviving a dose of giardia, I have used Pour n Go on a yearly basis for general disinfection, especially if there is no filtering.

 

But if the water smells like something has died in it, then a drain, clean and refill would be in order.

 

Rats can get stuck, and die, in the gutter and drainpipes. Makes the water taste funny.

 

 

Possums too... in the actual tank. "Thought it tasted a bit off..."  >.<