Hi,
Table salt (NaCl) should be avoided. The OP proposes to add a teaspoon per 15L-20L. In general, this is not good. Excessive Sodium (Na+) has negative effects by reducing K+ uptake. Excessive Chloride (Cl-) may have the effect of reducing NO3- uptake. Since Na+ plays a role in the osmotic pressure in plant cells, elevated Na+ will have the effect of disrupting control of the cells turgidity. Having too high of a salinity can set up a chain of events that negatively impacts the plant's metabolism and makes the plant cells less able to fend off injuries. There are other symptoms of salt stress, not the least of which is stunting.
This is also true when household water softened by salt resins is used in the tank, so this should be avoided if possible.
Cheers,