it really depends on the fish if they will be bothered by it or not, some like a lot of current and the drop in temp can trigger spawnings, however it can upset and even kill more delicate fish. can you tell us how big the tank is, what fish you have and how much water you change at any one time?
ways to slow down the current and the shock are
1 - pour the water in slower, requires a bit of control with your bucket arm! lol
2 - put the water in a jugfull at a time
3 - hold a plate just below the water surface and pour onto this, it spreads out the water and the flow
4 - use a hosepipe to fill the tank and control the speed with the tap
bogwood can lower pH however the effects can vary drastically from one piece of wood to another depending on a number of factors.