Hi lchaim, The general rule of thumb is "if it's good enough to drink, it's good enough for the TG". The general recommendation is to start with plain ol' tap water. The vast majority of TG growers find this to be their best choice. Unlike other water sources (ex: distilled), the chemistry of tap water is such that it typically contains pH buffer minerals. These buffers help stabilize pH. So your results are expected. If you don't already, the use of a sediment + carbon + KDF element filter is highly recommended regardless of your water source.