According to Doc Johnson, you want to be sure to obtain fish safe
aquaculture (not the penta-anhydrous form of photo hypo). I just bought
the
50 pound container from Aquatic Eco for $52 plus shipping, which should
treat about 1.5 million gallons of water. That should last me for a few
years. Compared to the about $50 per gallon for material to treat 37,000
gallons, that I went through about 4 of a year.
--

The Doc fails to tell us what the formula is for fish safe sodium
thiosulfate.

In our lab, we have Sodium Thiosulphate 5-Hydrate, which is hydrated rather
than anhydrous. This would be the logical alternative to the hydrated fish
harming variety, anybody know if this is what he recommends?
David