how much aragonite to add to an established 20 G goldfish tank?

Wow! Those levels are crazy. Your fish must be suffering:( So sorry you are dealing with this.

If it were me, I'd be ditching that water completely. I'd buy bottled spring water, or water from a local fish store. Like right now. That much ammonia will cause a lot of damage fast. That level of nitrate can cause poisoning and lead to swim bladder issues. What are your tap water parameters? You should test it immediately. It may not even be safe for you to drink.

This is from the World Health Organization (WHO):

"Below pH 4, redness and irritation of the eyes have been reported, the severity of which increases with decreasing pH. Below pH 2.5, damage to the epithelium is irreversible and extensive (10). In addition, because pH can affect the degree of corrosion of metals as well as disinfection efficiency, it may have an indirect effect on health."

There are no safe ammonia guidelines, but from what I have found, the higher the ammonia, the more faecal matter and impurities that are present in the water. Ew!

You need to do something fast if that ammonia is from the tap. If it is not, then your tank may have had it's cycle interrupted in the move. Big (50%) daily water changes are recommended until resolved. Not sure if the PH can be balanced enough. I've never heard of a PH of zero in tap water...

That said, your tank is too small for the fish. They may be creating more waste than the beneficial bacteria can handle. You need 40 gallons plus ten for additional fish (commons) and 30 gallons plus ten for additional fish (fancy).

Good luck!

If your water is truly hard, the pH can't possibly be zero. Perhaps your test kit is faulty.

Also, anything with a 0 pH (extreme acidity) would eat right through your pipes and would burn your skin if you touched the water. pH 0 is equivalent to concentrated hydrochloric acid.

It would take a lot of space to go into the chemistry of this, but please take my word for it.

Also, an ammonia level of 4.0 ppm is highly toxic, regardless of the pH.

Forget the aragonite until you can get the ammonia level down to near 0.0.

Aragonite probably won't harm the goldfish, but it's irrelevant to the ammonia problem.

Putting zeolite (also called ammo chips) instead of aragonite in those mesh bags might help (zeolite absorbs ammonia from fresh water).

But you should also find another source of water for your fish tank until you work out what's going on with your tap water chemistry.

Needless to say, it's also inadvisable to drink water (although water is not the right term for it) with a 0 pH, or even bathe in it or wash your hands with it.