I wasn't suggesting to lower the pH, or the softness/hardness or gH/kH, it is better to work with the natural conditions you have generally. You can adjust the gH/kH by using rainwater instead of tap water if you are on council water supply. You can add wood but that won't do much to the water once it's finished leaching tannins. I was basically suggesting that there aren't much options other than killing the snails (do a search for a range of suggestions), getting a snail eater (dwarf chain loaches would be ok if it's 21/22 degrees), or stripping out the gravel and baking it to kill the trumpet snails.