GH stands for General Hardness. General Hardness is a measure of dissolved minerals your water contains. Most of the minerals in your water will be calcium and Magnesium, however there will be others, but they will be in small amounts and not as significant. Depending on if you use Tap water or RO (Reverse Osmosis) water it will contain different levels of dissolved minerals. Understanding what GH you water has is a huge part of shrimp keeping.
GH test kits are not too difficult to use, however the hardest part is different parts of the world use different degrees of measurement. Here in the USA we measure 1 degree of GH as 17.9 parts per million (PPM). A huge mistake many new shrimp farmers make is not understanding or paying attention to their GH levels.
GH being especially important to shrimp keepers because Calcium Carbonate is vital to the molting cycle of shrimp. Molting is the key to shrimp growth and health. Think of drinking milk, it helps build strong bones in your body, it helps in the growth of your finger nails, hair…etc. Without enough Milk aka calcium we would not have strong bones…etc. Shrimp need their "Milk" aka calcium carbonate to aid in their molting process which is key to their lives. GH is a key player in breeding, often times if not kept in proper levels of GH shrimp will stop breeding all together.
Your aquarium shrimp is unable to grow at all without molting. Unlike us where we simply grow taller, shrimp are unable to grow unless they can molt successfully. Shrimp molt/shed their current smaller exoskeleton every so often so they can grow and form a newer large one. The rate at which a shrimp will molt/shed it exoskeleton has a lot to do with its environment and level of GH. The Calcium carbonate in the water is absorb by your shrimp and this is what creates new exoskeletons. With low levels of Calcium carbonate they will not develop proper shells and will have a harder time fighting off diseases, infections, and their overall health and immune system will not be up to par.
When talking about calcium carbonates you must also learn about magnesium as it is the key factor in making the calcium carbonate absorbable by your shrimp. If no magnesium was present in your aquarium your shrimp would suffer from lack of levels of calcium and soon after simply die off.
In closing you should take away from this article the fact that GH is one of the top 3 most important factors of keeping shrimp. Often times when searching on the web you will find people trying to understand why their shrimp simply died off. A key factor in die off in most new shrimp keeper's tanks is GH levels. So when keeping shrimp, test regularly for GH and make sure It is suitable for the type of shrimp you are keeping.
I personally use API GH &KH test kits as I find them to be the most reliable out on the market. Simply testing once every week or two will be suitable and might make the difference in saving your shrimp colony from die off!