Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I have heard of that method as described in post #1, and I disagree with his conclusion (There may be more than one person doing this method... but I think I know who you are talking about).
I think it is the accumulation of minerals (especially carbonates) because of topping off with tap water that is driving the pH up.
I see the same thing in my tanks in the summer. In the winter I top off with rain water, and do water changes with tap water. In the summer there is no rain, so I top off with tap water. TDS goes up over the summer, in inverse proportion to how many big water changes I do. When I do lots of water changes the TDS stays stable. The more topping off I do the more build up there is in the water.
Ditto m8e: There are many things that life does that lowers the pH. In addition to plants, microorganisms and animals also contribute to lowering the pH. In nature there is usually a balance, or else changes happen slowly, but look at how many soft water habitats there are, from rain forests to peat bogs. The hard water habitats are areas where more minerals are added to the water, such as the Rift Lakes, and waters in areas with high calcium and magnesium carbonate soils. Karst regions, for example.
While there sure might be minerals leaching from the gravel in those set ups, there is also peat moss blended in there. I know that peat moss can act like an ion-exchange water softener, but it eventually gets used up, I would think.