Is my tap water suitable for plants?Updated a month ago

Most aquatic plants grow better in soft water, but most species are tolerant of moderately hard water. Plants are generally much more sensitive to KH (alkalinity) than GH (General hardness). You can keep soft water plants in high GH, low KH water. However, you can't keep soft water plants in high KH water, no matter the GH.

As a general guide: 

Low KH - Can grow all species with adequate GH.

High KH - avoid soft water species.

Get a simple KH test kit- they are common and generally reliable. Local tap water reports will also contain information on your tap water's alkalinity.

Using the dKH scale:

Below 3 dKH - you have very soft water and you can grow species that require very soft water.

Between 3-8 dKH you can keep 97% of all commercial aquatic plants in optimal condition. Avoid sensitive soft water species such as Toninas, Syngonanthus, Ammania, sensitive Eriocaulons.

8 dKH and above - some Rotala macrandra varietals, sensitive Limnophilas and Bacopas species will not grow in optimal form in addition to the list above.

20 dKH and above - better to focus on plants that are fully in the hard water category.

Moderately hard water is not a big barrier to success. This tank measures around 8-9 dKH due to the limestone (Seiryu rock) used.