Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
No, dead snails do not raise pH significantly. The composting of their body lowers the pH, and the shell could raise the pH. Net is no significant change.
What may be happening is that the Aquasoil has removed all the carbonates it is capable of holding, so the carbonates are staying in the water, and this is keeping the pH up a bit.
pH is not a stand alone value. It is controlled by many of the minerals and salts in the tank, which can act as buffers, stabilizing the pH at a particular level. In aquariums one of the biggest buffers is carbonates. High carbonates usually means high pH that is difficult to change. Low carbonates generally means something else is going to control the pH. pH may be low from several acidic reactions going on- CO2, decomposing things, peat or driftwood, leaves (including oak, Indian Almond, other).
Several of the Aquasoil substrates, and some others (such as montmorillonite clays) can remove quite a bit of the carbonates from the water. This allows the pH to drop. (In some of my tanks it took laboratory grade tests to show the pH was pretty close to 5). These substrates can only hold a certain amount of carbonates. When they get full, they no longer remove carbonates from the water.
The carbonates remain in the water, and this controls the pH, and it rises.
More important than pH is the GH of the water.
Soft water fish evolved in water with very low mineral levels. Test the GH and keep it in the preferred range for your most particular fish.
Then adjust the KH so that the pH is in the right range.
When I was breeding Rams I used a blend of reverse osmosis and tap water to create GH and KH of about 2 German degrees of hardness, then filtered the water through peat moss overnight to drop the pH and add tannins that black water fish need. End result was pH in the low to mid 6s, and baby Rams.