GH (general hardness) of 150 ppm is ~8 dH on the so-called German scale, which is "moderately hard". Unfortunately all your mineral content seems to come from sulphates, because your carbonate hardness (KH, alkalinity) is 0 (a bare minimum level for stable pH would be a KH of 1 (~20 ppm), and 3 (~60 ppm) is generally recommended).
In your case the water has no buffering capacity to speak of. Did you test your tap water too? If that has a KH of 0 too, then feel free to go crazy with the coral sand and add as much of it as can fit in that mesh bag. If my tap water's KH was 0, I'd just use coral sand as substrate to be honest. But for the short term, I'd try to fix the water with bicarbonate, since it'll take a while for the coral sand to dissolve enough to provide decent buffering capacity.
Bicarbonate is a great tool if you know what you're doing, but it can be tricky stuff in the sense that it can change your pH unexpectedly fast like waterdrop said. The speed of the change could be hazardrous to your fish, but then, so are the current conditions in your tank. The silver lining on this particular cloud is that small amounts of bicarbonate will never raise your pH to a level that's lethal in itself (a 0.8% - or 8000 ppm - solution of sodium bicarbonate has a pH of 8.3; you'd need to dump an entire liter of baking soda in your tank to get there). It can, however, get your pH to ~8 at low concentrations. If one teaspoon isn't enough I'd add another (and so on), but I'd do it like this:
Make a stock solution of bicarbonate by dissolving 1 tsp of baking soda with ~1 liter of water in a bottle or whatever is handy. Add this to your tank slowly over an hour or so, ½ cup at a time, to minimize the shock to your fish. Measure your pH after another hour or so (to ensure the tank water has thoroughly mixed). If your pH still isn't in the reasonable range of 6.5 - 7.5, repeat the treatment and measure again. Doing this will give you a pretty good feel of exactly how much baking soda you need to adjust the pH of your tank in the future.
Do not use any commercial pH buffering chemicals if you're adjusting your water with bicarbonate.
Just be aware that any rise in pH can kill your fish at this point if you're unlucky. You have ammonia in the water, and ammonia becomes more toxic as pH rises. On the other hand, low pH prevents your filter bacteria from functioning properly, so you'll only get more ammonia if the pH isn't fixed. It's a nasty situation and I don't envy you.
