Here is my formula for re-mineralising RO or Rain water:   Powdered compound Grams needed: Calcium Sulphate CaSO4 Heptahydrate (so it dissolves easily) or Dihydrate if you can't find it, but dihydrate is not as soluble. 58gm Magnesium Sulphate (Epsom Salt) MgSO4 37gm Potassium Sulphate (aka Sulphate Of Potash) K2SO4 11gm Iron Sulphate (optional) FeSO4 0.30gm Manganese Sulphate (optional) MnSO4 0.16gm Total weight  106.46gm Multiply accordingly if you need to mix bigger batches.   Alternatively, Iron and Manganese can be replaced with a Micro-nutrient (trace element) mix of 0.46gm. Again micro nutrients or trace elements are optional, but a small amount of these minerals can be beneficial for shrimps. They will get it elsewhere if it is not added here.   You can either use it in powder form, adding small teaspoons to your water change until a TDS of 140-160 is reached. OR You can premix this in 500ml bottle of RO water, and drip it into your water change until the desired TDS 140-160 is reached.   If you have a TDS pen, you can check how much 1gm will raise TDS in 1L of water. Similarly you can also test GH/KH raise in 1L of water.   It is very important you test this yourself, since there can be a number of variables between your mix and my mix.   This mix will not alter pH.   You will be able to find all you need at    <--- Back in business as of Jan 2018! It is getting more difficult sourcing Calcium Sulphate heptahydrate. The best source I can find is from home brew shops. - $9.75 for a kilo. This is food grade stuff. removed - never seem to be in stock. - $4.95 for 300g.  removed - never seem to be in stock. - $20.89 for 1kg of Calcium Sulphate   Alternatively, if you need a remin mix for Neocaridina or Tigers, you can replace 15g - 20g of Calcium Sulphate with Calcium Carbonate. This will raise KH a bit. - $4.95 for 300g of Calcium Carbonate.