Okay I'm no expert but I used to be a plumber..
Now with them toilets the cistern drains down then fills back up (fish are inside the cistern)
Now IF the cistern is attached the the MAINS water supply, they will be filled back up with Water full with chlorine. (now I'm sure the toilet is flushed more than once) which is obviously no good, because adding chlorine would be pointless.
However if the toilet is filled from a Tank in the loft
that water sits stagnant in the tank for some time. (until a bath is run or toilet flushed) these tanks are large so 90% of the water will be stale and nasty (yes chlorine will be removed over time) but have you ever seen inside one of them?
all the crap that accumulates at the bottom? I have seen dead birds and polystyrene inside there.
gunk that you can pick up.
So I would assume its not the most nice experience of all.
I think it's a disgrace, Although yes they would get frequent water changes....
Although you can get ones that are separate from the actual cistern they still are horrible IMO