Our local hardware seems bags of the large rock crystals. Looks like salt, in lumps the size of your fist. We were told it was to make tank water clear. Does anyone have any information on what this might be or if it works?

Likely it is "alum":

https://en.wikipedia.org/wiki/Alum

The German WiKi article explicitly mentions that in Thailand it is used to clear water from particulate material / sediments.

(the author must be an expat )

An "educational" video in Thai language:

(alum in Thai "sansom")

Edited November 4, 201510 yr by KhunBENQ

Unless this stuff can be positively identified do not dump it in water tanks.

Looks like it for sure. Thank you

Another interesting use of alum.

I used it sometimes under the armpit

I will inquire the Thai family/shop owners and give it a try in our water tank.

(we use this water only for shower, cleaning, flushing)

Just Google for "alum water treatment" and find a lot of documention and videos (in English).

Edited November 4, 201510 yr by KhunBENQ

We only use for this too. We're 90% rain water but in a pinch sometimes need to use town / mains water and it can be dirty sometimes