Many people now have easy access to Reverse Osmosis (RO) water either brought from their local fish shop or via a RO unit at home. This RO water is almost completely void of any minerals or compounds that are required for healthy plants. Normally either some tap water or one of the many products that are on the market can be used to remineralise the RO water to make it suitable for plants. There are pros and cons to both these methods.
Tap water is cheap to use. Main problem being is that you aren't sure of what is being added. Levels of hardness and other compounds can also vary throughout the year.
Remineralising agents aren't always aimed at planted tanks and when they are I don't find them to be that good and push TDS very high for even a small rise in GH. They can be quite expensive for what they are. Also they normally don't contain any buffering compounds which have to be purchased separately.

Rather than buying a premade product you can buy the individual components and make your own remineralising agent to suit your own needs.

Alkalinity

Another component that needs to be thought about is the Alkalinity (buffering capability), or often wrongly referred to as KH. It is actually possible to run planted tanks with zero alkalinity as long as you do plenty of water changes to prevent any build-up of acid. I like to add a small amount of buffering to my tanks using potassium carbonate. It is often suggested to use sodium bicarbonate to increase alkalinity, but I think this is a bad idea as too much sodium can be introduced into the tank. For example raising the alkalinity by 4 would add 33ppm of sodium, whilst not that high, it is higher than I would like. For this reason I use potassium carbonate which is easily available.

When you hear of some plants not liking hard water, what is really meant is that they don't like high alkalinity. They however do like Calcium and Magnesium which makes up part of the GH. Plants require GH to grow but not alkalinity.

DIY Remineralising Agent

There are many different formulations on the web of how to make your own remineralising agent but none really suited my needs so I decided to devise my own formulation. My objectives were having a low KH, a 3:1 ratio of calcium and magnesium providing a good level of GH, no sodium and a small amount of chloride. This is what I came up with:

0.4g Calcium Chloride Dihydrate
2.0g Calcium Sulphate Dihydrate
2.0g Magnesium Sulphate Heptahydrate
0.7g Potassium Carbonate
25 litres Water

The above solution gives:
24ppm Calcium
8ppm Magnesium
16ppm Potassium
7ppm Chloride
GH=5
KH=1

I find that these levels suit me very well but can always be tweaked if desired to suit your particular needs.

The KH is very low as I find some plants seem to thrive with these low levels. As long as your weekly water changes are maintained there shouldn't be any problems with having a very low KH even when injecting CO2.

The chemicals listed above are the most commonly sold types that I've come across, so if when purchasing it just states calcium sulphate rather than calcium sulphate dihydrate, then the chances are that it would be the dihydrate form.

Notes

1 dGH = 7.2mg/l Ca++
1 dGH = 4.3mg/l Mg++

1.5g NaHCO3 in 25 litres of water = 2 dKH
1.8g KHCO3 in 25 litres of water = 2 dKH
1.2g K2CO3 in 25 litres of water = 2 dKH

Calcium Sulphate, Magnesium Sulphate and Potassium Carbonate can be purchased from The Nutrient Company. Calcium Chloride can normally be purchased off eBay as it's used in many reef tanks.