Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Highly likely that is also in mg/l.
There are 2 more common ways that hardness (both sorts) are reported in the aquarium hobby.
milligrams per liter is the same as parts per million. It is usually stated as mg/l in the metric system.
German degrees of hardness is the other common way of showing it. There are 17.9 mg/l in 1 degree.
138.8 mg/l = 7.75 German degrees of hardness.
There are 2 types of hardness commonly measured in aquariums.
GH is General Hardness. It is a measure of the calcium and magnesium levels. This is the most important one for fish.
KH is Carbonate Hardness. It is a measure of carbonates and bicarbonates. These act like a buffer and can stabilize the pH. High KH generally means the pH will be high and hard to adjust. Low KH allows something else in the water to control the pH. The 'something else' may be something added to the water by the water company to keep the water more alkaline, or it might be organic matter (peat moss, driftwood) added to the tank or filter. Organic matter usually makes the water more acidic.
To make the water right for the fish, the first steps are to research the fish, and investigate the water you will be starting with.
I first make the GH right for the fish I want in that tank.
Then make the KH more or less match the GH.
The pH usually takes care of itself. I would add peat moss for fish from black water streams or lakes.